ip-cref.tex revision e2613dc8605e56dbc53890ebbae263f93610bd41
1\documentstyle[12pt,twoside]{article} 2\def\TITLE{IP Command Reference} 3\input preamble 4\begin{center} 5\Large\bf IP Command Reference. 6\end{center} 7 8 9\begin{center} 10{ \large Alexey~N.~Kuznetsov } \\ 11\em Institute for Nuclear Research, Moscow \\ 12\verb|kuznet@ms2.inr.ac.ru| \\ 13\rm April 14, 1999 14\end{center} 15 16\vspace{5mm} 17 18\tableofcontents 19 20\newpage 21 22\section{About this document} 23 24This document presents a comprehensive description of the \verb|ip| utility 25from the \verb|iproute2| package. It is not a tutorial or user's guide. 26It is a {\em dictionary\/}, not explaining terms, 27but translating them into other terms, which may also be unknown to the reader. 28However, the document is self-contained and the reader, provided they have a 29basic networking background, will find enough information 30and examples to understand and configure Linux-2.2 IP and IPv6 31networking. 32 33This document is split into sections explaining \verb|ip| commands 34and options, decrypting \verb|ip| output and containing a few examples. 35More voluminous examples and some topics, which require more elaborate 36discussion, are in the appendix. 37 38The paragraphs beginning with NB contain side notes, warnings about 39bugs and design drawbacks. They may be skipped at the first reading. 40 41\section{{\tt ip} --- command syntax} 42 43The generic form of an \verb|ip| command is: 44\begin{verbatim} 45ip [ OPTIONS ] OBJECT [ COMMAND [ ARGUMENTS ]] 46\end{verbatim} 47where \verb|OPTIONS| is a set of optional modifiers affecting the 48general behaviour of the \verb|ip| utility or changing its output. All options 49begin with the character \verb|'-'| and may be used in either long or abbreviated 50forms. Currently, the following options are available: 51 52\begin{itemize} 53\item \verb|-V|, \verb|-Version| 54 55--- print the version of the \verb|ip| utility and exit. 56 57 58\item \verb|-s|, \verb|-stats|, \verb|-statistics| 59 60--- output more information. If the option 61appears twice or more, the amount of information increases. 62As a rule, the information is statistics or some time values. 63 64 65\item \verb|-f|, \verb|-family| followed by a protocol family 66identifier: \verb|inet|, \verb|inet6| or \verb|link|. 67 68--- enforce the protocol family to use. If the option is not present, 69the protocol family is guessed from other arguments. If the rest of the command 70line does not give enough information to guess the family, \verb|ip| falls back to the default 71one, usually \verb|inet| or \verb|any|. \verb|link| is a special family 72identifier meaning that no networking protocol is involved. 73 74\item \verb|-4| 75 76--- shortcut for \verb|-family inet|. 77 78\item \verb|-6| 79 80--- shortcut for \verb|-family inet6|. 81 82\item \verb|-0| 83 84--- shortcut for \verb|-family link|. 85 86 87\item \verb|-o|, \verb|-oneline| 88 89--- output each record on a single line, replacing line feeds 90with the \verb|'\'| character. This is convenient when you want to 91count records with \verb|wc| or to \verb|grep| the output. The trivial 92script \verb|rtpr| converts the output back into readable form. 93 94\item \verb|-r|, \verb|-resolve| 95 96--- use the system's name resolver to print DNS names instead of 97host addresses. 98 99\begin{NB} 100 Do not use this option when reporting bugs or asking for advice. 101\end{NB} 102\begin{NB} 103 \verb|ip| never uses DNS to resolve names to addresses. 104\end{NB} 105 106\end{itemize} 107 108\verb|OBJECT| is the object to manage or to get information about. 109The object types currently understood by \verb|ip| are: 110 111\begin{itemize} 112\item \verb|link| --- network device 113\item \verb|address| --- protocol (IP or IPv6) address on a device 114\item \verb|neighbour| --- ARP or NDISC cache entry 115\item \verb|route| --- routing table entry 116\item \verb|rule| --- rule in routing policy database 117\item \verb|maddress| --- multicast address 118\item \verb|mroute| --- multicast routing cache entry 119\item \verb|tunnel| --- tunnel over IP 120\end{itemize} 121 122Again, the names of all objects may be written in full or 123abbreviated form, f.e.\ \verb|address| is abbreviated as \verb|addr| 124or just \verb|a|. 125 126\verb|COMMAND| specifies the action to perform on the object. 127The set of possible actions depends on the object type. 128As a rule, it is possible to \verb|add|, \verb|delete| and 129\verb|show| (or \verb|list|) objects, but some objects 130do not allow all of these operations or have some additional commands. 131The \verb|help| command is available for all objects. It prints 132out a list of available commands and argument syntax conventions. 133 134If no command is given, some default command is assumed. 135Usually it is \verb|list| or, if the objects of this class 136cannot be listed, \verb|help|. 137 138\verb|ARGUMENTS| is a list of arguments to the command. 139The arguments depend on the command and object. There are two types of arguments: 140{\em flags\/}, consisting of a single keyword, and {\em parameters\/}, 141consisting of a keyword followed by a value. For convenience, 142each command has some {\em default parameter\/} 143which may be omitted. F.e.\ parameter \verb|dev| is the default 144for the {\tt ip link} command, so {\tt ip link ls eth0} is equivalent 145to {\tt ip link ls dev eth0}. 146In the command descriptions below such parameters 147are distinguished with the marker: ``(default)''. 148 149Almost all keywords may be abbreviated with several first (or even single) 150letters. The shortcuts are convenient when \verb|ip| is used interactively, 151but they are not recommended in scripts or when reporting bugs 152or asking for advice. ``Officially'' allowed abbreviations are listed 153in the document body. 154 155 156 157\section{{\tt ip} --- error messages} 158 159\verb|ip| may fail for one of the following reasons: 160 161\begin{itemize} 162\item 163A syntax error on the command line: an unknown keyword, incorrectly formatted 164IP address {\em et al\/}. In this case \verb|ip| prints an error message 165and exits. As a rule, the error message will contain information 166about the reason for the failure. Sometimes it also prints a help page. 167 168\item 169The arguments did not pass verification for self-consistency. 170 171\item 172\verb|ip| failed to compile a kernel request from the arguments 173because the user didn't give enough information. 174 175\item 176The kernel returned an error to some syscall. In this case \verb|ip| 177prints the error message, as it is output with \verb|perror(3)|, 178prefixed with a comment and a syscall identifier. 179 180\item 181The kernel returned an error to some RTNETLINK request. 182In this case \verb|ip| prints the error message, as it is output 183with \verb|perror(3)| prefixed with ``RTNETLINK answers:''. 184 185\end{itemize} 186 187All the operations are atomic, i.e.\ 188if the \verb|ip| utility fails, it does not change anything 189in the system. One harmful exception is \verb|ip link| command 190(Sec.\ref{IP-LINK}, p.\pageref{IP-LINK}), 191which may change only some of the device parameters given 192on command line. 193 194It is difficult to list all the error messages (especially 195syntax errors). However, as a rule, their meaning is clear 196from the context of the command. 197 198The most common mistakes are: 199 200\begin{enumerate} 201\item Netlink is not configured in the kernel. The message is: 202\begin{verbatim} 203Cannot open netlink socket: Invalid value 204\end{verbatim} 205 206\item RTNETLINK is not configured in the kernel. In this case 207one of the following messages may be printed, depending on the command: 208\begin{verbatim} 209Cannot talk to rtnetlink: Connection refused 210Cannot send dump request: Connection refused 211\end{verbatim} 212 213\item The \verb|CONFIG_IP_MULTIPLE_TABLES| option was not selected 214when configuring the kernel. In this case any attempt to use the 215\verb|ip| \verb|rule| command will fail, f.e. 216\begin{verbatim} 217kuznet@kaiser $ ip rule list 218RTNETLINK error: Invalid argument 219dump terminated 220\end{verbatim} 221 222\end{enumerate} 223 224 225\section{{\tt ip link} --- network device configuration} 226\label{IP-LINK} 227 228\paragraph{Object:} A \verb|link| is a network device and the corresponding 229commands display and change the state of devices. 230 231\paragraph{Commands:} \verb|set| and \verb|show| (or \verb|list|). 232 233\subsection{{\tt ip link set} --- change device attributes} 234 235\paragraph{Abbreviations:} \verb|set|, \verb|s|. 236 237\paragraph{Arguments:} 238 239\begin{itemize} 240\item \verb|dev NAME| (default) 241 242--- \verb|NAME| specifies the network device on which to operate. 243 244\item \verb|up| and \verb|down| 245 246--- change the state of the device to \verb|UP| or \verb|DOWN|. 247 248\item \verb|arp on| or \verb|arp off| 249 250--- change the \verb|NOARP| flag on the device. 251 252\begin{NB} 253This operation is {\em not allowed\/} if the device is in state \verb|UP|. 254Though neither the \verb|ip| utility nor the kernel check for this condition. 255You can get unpredictable results changing this flag while the 256device is running. 257\end{NB} 258 259\item \verb|multicast on| or \verb|multicast off| 260 261--- change the \verb|MULTICAST| flag on the device. 262 263\item \verb|dynamic on| or \verb|dynamic off| 264 265--- change the \verb|DYNAMIC| flag on the device. 266 267\item \verb|name NAME| 268 269--- change the name of the device. This operation is not 270recommended if the device is running or has some addresses 271already configured. 272 273\item \verb|txqueuelen NUMBER| or \verb|txqlen NUMBER| 274 275--- change the transmit queue length of the device. 276 277\item \verb|mtu NUMBER| 278 279--- change the MTU of the device. 280 281\item \verb|address LLADDRESS| 282 283--- change the station address of the interface. 284 285\item \verb|broadcast LLADDRESS|, \verb|brd LLADDRESS| or \verb|peer LLADDRESS| 286 287--- change the link layer broadcast address or the peer address when 288the interface is \verb|POINTOPOINT|. 289 290\vskip 1mm 291\begin{NB} 292For most devices (f.e.\ for Ethernet) changing the link layer 293broadcast address will break networking. 294Do not use it, if you do not understand what this operation really does. 295\end{NB} 296 297\item \verb|netns PID| 298 299--- move the device to the network namespace associated with the process PID. 300 301\end{itemize} 302 303\vskip 1mm 304\begin{NB} 305The \verb|PROMISC| and \verb|ALLMULTI| flags are considered 306obsolete and should not be changed administratively, though 307the {\tt ip} utility will allow that. 308\end{NB} 309 310\paragraph{Warning:} If multiple parameter changes are requested, 311\verb|ip| aborts immediately after any of the changes have failed. 312This is the only case when \verb|ip| can move the system to 313an unpredictable state. The solution is to avoid changing 314several parameters with one {\tt ip link set} call. 315 316\paragraph{Examples:} 317\begin{itemize} 318\item \verb|ip link set dummy address 00:00:00:00:00:01| 319 320--- change the station address of the interface \verb|dummy|. 321 322\item \verb|ip link set dummy up| 323 324--- start the interface \verb|dummy|. 325 326\end{itemize} 327 328 329\subsection{{\tt ip link show} --- display device attributes} 330\label{IP-LINK-SHOW} 331 332\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|lst|, \verb|sh|, \verb|ls|, 333\verb|l|. 334 335\paragraph{Arguments:} 336\begin{itemize} 337\item \verb|dev NAME| (default) 338 339--- \verb|NAME| specifies the network device to show. 340If this argument is omitted all devices are listed. 341 342\item \verb|up| 343 344--- only display running interfaces. 345 346\end{itemize} 347 348 349\paragraph{Output format:} 350 351\begin{verbatim} 352kuznet@alisa:~ $ ip link ls eth0 3533: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc cbq qlen 100 354 link/ether 00:a0:cc:66:18:78 brd ff:ff:ff:ff:ff:ff 355kuznet@alisa:~ $ ip link ls sit0 3565: sit0@NONE: <NOARP,UP> mtu 1480 qdisc noqueue 357 link/sit 0.0.0.0 brd 0.0.0.0 358kuznet@alisa:~ $ ip link ls dummy 3592: dummy: <BROADCAST,NOARP> mtu 1500 qdisc noop 360 link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff 361kuznet@alisa:~ $ 362\end{verbatim} 363 364 365The number before each colon is an {\em interface index\/} or {\em ifindex\/}. 366This number uniquely identifies the interface. This is followed by the {\em interface name\/} 367(\verb|eth0|, \verb|sit0| etc.). The interface name is also 368unique at every given moment. However, the interface may disappear from the 369list (f.e.\ when the corresponding driver module is unloaded) and another 370one with the same name may be created later. Besides that, 371the administrator may change the name of any device with 372\verb|ip| \verb|link| \verb|set| \verb|name| 373to make it more intelligible. 374 375The interface name may have another name or \verb|NONE| appended 376after the \verb|@| sign. This means that this device is bound to some other 377device, 378i.e.\ packets send through it are encapsulated and sent via the ``master'' 379device. If the name is \verb|NONE|, the master is unknown. 380 381Then we see the interface {\em mtu\/} (``maximal transfer unit''). This determines 382the maximal size of data which can be sent as a single packet over this interface. 383 384{\em qdisc\/} (``queuing discipline'') shows the queuing algorithm used 385on the interface. Particularly, \verb|noqueue| means that this interface 386does not queue anything and \verb|noop| means that the interface is in blackhole 387mode i.e.\ all packets sent to it are immediately discarded. 388{\em qlen\/} is the default transmit queue length of the device measured 389in packets. 390 391The interface flags are summarized in the angle brackets. 392 393\begin{itemize} 394\item \verb|UP| --- the device is turned on. It is ready to accept 395packets for transmission and it may inject into the kernel packets received 396from other nodes on the network. 397 398\item \verb|LOOPBACK| --- the interface does not communicate with other 399hosts. All packets sent through it will be returned 400and nothing but bounced packets can be received. 401 402\item \verb|BROADCAST| --- the device has the facility to send packets 403to all hosts sharing the same link. A typical example is an Ethernet link. 404 405\item \verb|POINTOPOINT| --- the link has only two ends with one node 406attached to each end. All packets sent to this link will reach the peer 407and all packets received by us came from this single peer. 408 409If neither \verb|LOOPBACK| nor \verb|BROADCAST| nor \verb|POINTOPOINT| 410are set, the interface is assumed to be NMBA (Non-Broadcast Multi-Access). 411This is the most generic type of device and the most complicated one, because 412the host attached to a NBMA link has no means to send to anyone 413without additionally configured information. 414 415\item \verb|MULTICAST| --- is an advisory flag indicating that the interface 416is aware of multicasting i.e.\ sending packets to some subset of neighbouring 417nodes. Broadcasting is a particular case of multicasting, where the multicast 418group consists of all nodes on the link. It is important to emphasize 419that software {\em must not\/} interpret the absence of this flag as the inability 420to use multicasting on this interface. Any \verb|POINTOPOINT| and 421\verb|BROADCAST| link is multicasting by definition, because we have 422direct access to all the neighbours and, hence, to any part of them. 423Certainly, the use of high bandwidth multicast transfers is not recommended 424on broadcast-only links because of high expense, but it is not strictly 425prohibited. 426 427\item \verb|PROMISC| --- the device listens to and feeds to the kernel all 428traffic on the link even if it is not destined for us, not broadcasted 429and not destined for a multicast group of which we are member. Usually 430this mode exists only on broadcast links and is used by bridges and for network 431monitoring. 432 433\item \verb|ALLMULTI| --- the device receives all multicast packets 434wandering on the link. This mode is used by multicast routers. 435 436\item \verb|NOARP| --- this flag is different from the other ones. It has 437no invariant value and its interpretation depends on the network protocols 438involved. As a rule, it indicates that the device needs no address 439resolution and that the software or hardware knows how to deliver packets 440without any help from the protocol stacks. 441 442\item \verb|DYNAMIC| --- is an advisory flag indicating that the interface is 443dynamically created and destroyed. 444 445\item \verb|SLAVE| --- this interface is bonded to some other interfaces 446to share link capacities. 447 448\end{itemize} 449 450\vskip 1mm 451\begin{NB} 452There are other flags but they are either obsolete (\verb|NOTRAILERS|) 453or not implemented (\verb|DEBUG|) or specific to some devices 454(\verb|MASTER|, \verb|AUTOMEDIA| and \verb|PORTSEL|). We do not discuss 455them here. 456\end{NB} 457 458 459The second line contains information on the link layer addresses 460associated with the device. The first word (\verb|ether|, \verb|sit|) 461defines the interface hardware type. This type determines the format and semantics 462of the addresses and is logically part of the address. 463The default format of the station address and the broadcast address 464(or the peer address for pointopoint links) is a 465sequence of hexadecimal bytes separated by colons, but some link 466types may have their natural address format, f.e.\ addresses 467of tunnels over IP are printed as dotted-quad IP addresses. 468 469\vskip 1mm 470\begin{NB} 471 NBMA links have no well-defined broadcast or peer address, 472 however this field may contain useful information, f.e.\ 473 about the address of broadcast relay or about the address of the ARP server. 474\end{NB} 475\begin{NB} 476Multicast addresses are not shown by this command, see 477\verb|ip maddr ls| in~Sec.\ref{IP-MADDR} (p.\pageref{IP-MADDR} of this 478document). 479\end{NB} 480 481 482\paragraph{Statistics:} With the \verb|-statistics| option, \verb|ip| also 483prints interface statistics: 484 485\begin{verbatim} 486kuznet@alisa:~ $ ip -s link ls eth0 4873: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc cbq qlen 100 488 link/ether 00:a0:cc:66:18:78 brd ff:ff:ff:ff:ff:ff 489 RX: bytes packets errors dropped overrun mcast 490 2449949362 2786187 0 0 0 0 491 TX: bytes packets errors dropped carrier collsns 492 178558497 1783945 332 0 332 35172 493kuznet@alisa:~ $ 494\end{verbatim} 495\verb|RX:| and \verb|TX:| lines summarize receiver and transmitter 496statistics. They contain: 497\begin{itemize} 498\item \verb|bytes| --- the total number of bytes received or transmitted 499on the interface. This number wraps when the maximal length of the data type 500natural for the architecture is exceeded, so continuous monitoring requires 501a user level daemon snapping it periodically. 502\item \verb|packets| --- the total number of packets received or transmitted 503on the interface. 504\item \verb|errors| --- the total number of receiver or transmitter errors. 505\item \verb|dropped| --- the total number of packets dropped due to lack 506of resources. 507\item \verb|overrun| --- the total number of receiver overruns resulting 508in dropped packets. As a rule, if the interface is overrun, it means 509serious problems in the kernel or that your machine is too slow 510for this interface. 511\item \verb|mcast| --- the total number of received multicast packets. This option 512is only supported by a few devices. 513\item \verb|carrier| --- total number of link media failures f.e.\ because 514of lost carrier. 515\item \verb|collsns| --- the total number of collision events 516on Ethernet-like media. This number may have a different sense on other 517link types. 518\item \verb|compressed| --- the total number of compressed packets. This is 519available only for links using VJ header compression. 520\end{itemize} 521 522 523If the \verb|-s| option is entered twice or more, 524\verb|ip| prints more detailed statistics on receiver 525and transmitter errors. 526 527\begin{verbatim} 528kuznet@alisa:~ $ ip -s -s link ls eth0 5293: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc cbq qlen 100 530 link/ether 00:a0:cc:66:18:78 brd ff:ff:ff:ff:ff:ff 531 RX: bytes packets errors dropped overrun mcast 532 2449949362 2786187 0 0 0 0 533 RX errors: length crc frame fifo missed 534 0 0 0 0 0 535 TX: bytes packets errors dropped carrier collsns 536 178558497 1783945 332 0 332 35172 537 TX errors: aborted fifo window heartbeat 538 0 0 0 332 539kuznet@alisa:~ $ 540\end{verbatim} 541These error names are pure Ethernetisms. Other devices 542may have non zero values in these fields but they may be 543interpreted differently. 544 545 546\section{{\tt ip address} --- protocol address management} 547 548\paragraph{Abbreviations:} \verb|address|, \verb|addr|, \verb|a|. 549 550\paragraph{Object:} The \verb|address| is a protocol (IP or IPv6) address attached 551to a network device. Each device must have at least one address 552to use the corresponding protocol. It is possible to have several 553different addresses attached to one device. These addresses are not 554discriminated, so that the term {\em alias\/} is not quite appropriate 555for them and we do not use it in this document. 556 557The \verb|ip addr| command displays addresses and their properties, 558adds new addresses and deletes old ones. 559 560\paragraph{Commands:} \verb|add|, \verb|delete|, \verb|flush| and \verb|show| 561(or \verb|list|). 562 563 564\subsection{{\tt ip address add} --- add a new protocol address} 565\label{IP-ADDR-ADD} 566 567\paragraph{Abbreviations:} \verb|add|, \verb|a|. 568 569\paragraph{Arguments:} 570 571\begin{itemize} 572\item \verb|dev NAME| 573 574\noindent--- the name of the device to add the address to. 575 576\item \verb|local ADDRESS| (default) 577 578--- the address of the interface. The format of the address depends 579on the protocol. It is a dotted quad for IP and a sequence of hexadecimal halfwords 580separated by colons for IPv6. The \verb|ADDRESS| may be followed by 581a slash and a decimal number which encodes the network prefix length. 582 583 584\item \verb|peer ADDRESS| 585 586--- the address of the remote endpoint for pointopoint interfaces. 587Again, the \verb|ADDRESS| may be followed by a slash and a decimal number, 588encoding the network prefix length. If a peer address is specified, 589the local address {\em cannot\/} have a prefix length. The network prefix is associated 590with the peer rather than with the local address. 591 592 593\item \verb|broadcast ADDRESS| 594 595--- the broadcast address on the interface. 596 597It is possible to use the special symbols \verb|'+'| and \verb|'-'| 598instead of the broadcast address. In this case, the broadcast address 599is derived by setting/resetting the host bits of the interface prefix. 600 601\vskip 1mm 602\begin{NB} 603Unlike \verb|ifconfig|, the \verb|ip| utility {\em does not\/} set any broadcast 604address unless explicitly requested. 605\end{NB} 606 607 608\item \verb|label NAME| 609 610--- Each address may be tagged with a label string. 611In order to preserve compatibility with Linux-2.0 net aliases, 612this string must coincide with the name of the device or must be prefixed 613with the device name followed by colon. 614 615 616\item \verb|scope SCOPE_VALUE| 617 618--- the scope of the area where this address is valid. 619The available scopes are listed in file \verb|/etc/iproute2/rt_scopes|. 620Predefined scope values are: 621 622 \begin{itemize} 623 \item \verb|global| --- the address is globally valid. 624 \item \verb|site| --- (IPv6 only) the address is site local, 625 i.e.\ it is valid inside this site. 626 \item \verb|link| --- the address is link local, i.e.\ 627 it is valid only on this device. 628 \item \verb|host| --- the address is valid only inside this host. 629 \end{itemize} 630 631Appendix~\ref{ADDR-SEL} (p.\pageref{ADDR-SEL} of this document) 632contains more details on address scopes. 633 634\end{itemize} 635 636\paragraph{Examples:} 637\begin{itemize} 638\item \verb|ip addr add 127.0.0.1/8 dev lo brd + scope host| 639 640--- add the usual loopback address to the loopback device. 641 642\item \verb|ip addr add 10.0.0.1/24 brd + dev eth0 label eth0:Alias| 643 644--- add the address 10.0.0.1 with prefix length 24 (i.e.\ netmask 645\verb|255.255.255.0|), standard broadcast and label \verb|eth0:Alias| 646to the interface \verb|eth0|. 647\end{itemize} 648 649 650\subsection{{\tt ip address delete} --- delete a protocol address} 651 652\paragraph{Abbreviations:} \verb|delete|, \verb|del|, \verb|d|. 653 654\paragraph{Arguments:} coincide with the arguments of \verb|ip addr add|. 655The device name is a required argument. The rest are optional. 656If no arguments are given, the first address is deleted. 657 658\paragraph{Examples:} 659\begin{itemize} 660\item \verb|ip addr del 127.0.0.1/8 dev lo| 661 662--- deletes the loopback address from the loopback device. 663It would be best not to repeat this experiment. 664 665\item Disable IP on the interface \verb|eth0|: 666\begin{verbatim} 667 while ip -f inet addr del dev eth0; do 668 : nothing 669 done 670\end{verbatim} 671Another method to disable IP on an interface using {\tt ip addr flush} 672may be found in sec.\ref{IP-ADDR-FLUSH}, p.\pageref{IP-ADDR-FLUSH}. 673 674\end{itemize} 675 676 677\subsection{{\tt ip address show} --- display protocol addresses} 678 679\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|lst|, \verb|sh|, \verb|ls|, 680\verb|l|. 681 682\paragraph{Arguments:} 683 684\begin{itemize} 685\item \verb|dev NAME| (default) 686 687--- the name of the device. 688 689\item \verb|scope SCOPE_VAL| 690 691--- only list addresses with this scope. 692 693\item \verb|to PREFIX| 694 695--- only list addresses matching this prefix. 696 697\item \verb|label PATTERN| 698 699--- only list addresses with labels matching the \verb|PATTERN|. 700\verb|PATTERN| is a usual shell style pattern. 701 702 703\item \verb|dynamic| and \verb|permanent| 704 705--- (IPv6 only) only list addresses installed due to stateless 706address configuration or only list permanent (not dynamic) addresses. 707 708\item \verb|tentative| 709 710--- (IPv6 only) only list addresses which did not pass duplicate 711address detection. 712 713\item \verb|deprecated| 714 715--- (IPv6 only) only list deprecated addresses. 716 717 718\item \verb|primary| and \verb|secondary| 719 720--- only list primary (or secondary) addresses. 721 722\end{itemize} 723 724 725\paragraph{Output format:} 726 727\begin{verbatim} 728kuznet@alisa:~ $ ip addr ls eth0 7293: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc cbq qlen 100 730 link/ether 00:a0:cc:66:18:78 brd ff:ff:ff:ff:ff:ff 731 inet 193.233.7.90/24 brd 193.233.7.255 scope global eth0 732 inet6 3ffe:2400:0:1:2a0:ccff:fe66:1878/64 scope global dynamic 733 valid_lft forever preferred_lft 604746sec 734 inet6 fe80::2a0:ccff:fe66:1878/10 scope link 735kuznet@alisa:~ $ 736\end{verbatim} 737 738The first two lines coincide with the output of \verb|ip link ls|. 739It is natural to interpret link layer addresses 740as addresses of the protocol family \verb|AF_PACKET|. 741 742Then the list of IP and IPv6 addresses follows, accompanied by 743additional address attributes: scope value (see Sec.\ref{IP-ADDR-ADD}, 744p.\pageref{IP-ADDR-ADD} above), flags and the address label. 745 746Address flags are set by the kernel and cannot be changed 747administratively. Currently, the following flags are defined: 748 749\begin{enumerate} 750\item \verb|secondary| 751 752--- the address is not used when selecting the default source address 753of outgoing packets (Cf.\ Appendix~\ref{ADDR-SEL}, p.\pageref{ADDR-SEL}.). 754An IP address becomes secondary if another address with the same 755prefix bits already exists. The first address is primary. 756It is the leader of the group of all secondary addresses. When the leader 757is deleted, all secondaries are purged too. 758There is a tweak in \verb|/proc/sys/net/ipv4/conf/<dev>/promote_secondaries| 759which activate secondaries promotion when a primary is deleted. 760To permanently enable this feature on all devices add 761\verb|net.ipv4.conf.all.promote_secondaries=1| to \verb|/etc/sysctl.conf|. 762This tweak is available in linux 2.6.15 and later. 763 764 765\item \verb|dynamic| 766 767--- the address was created due to stateless autoconfiguration~\cite{RFC-ADDRCONF}. 768In this case the output also contains information on times, when 769the address is still valid. After \verb|preferred_lft| expires the address is 770moved to the deprecated state. After \verb|valid_lft| expires the address 771is finally invalidated. 772 773\item \verb|deprecated| 774 775--- the address is deprecated, i.e.\ it is still valid, but cannot 776be used by newly created connections. 777 778\item \verb|tentative| 779 780--- the address is not used because duplicate address detection~\cite{RFC-ADDRCONF} 781is still not complete or failed. 782 783\end{enumerate} 784 785 786\subsection{{\tt ip address flush} --- flush protocol addresses} 787\label{IP-ADDR-FLUSH} 788 789\paragraph{Abbreviations:} \verb|flush|, \verb|f|. 790 791\paragraph{Description:}This command flushes the protocol addresses 792selected by some criteria. 793 794\paragraph{Arguments:} This command has the same arguments as \verb|show|. 795The difference is that it does not run when no arguments are given. 796 797\paragraph{Warning:} This command (and other \verb|flush| commands 798described below) is pretty dangerous. If you make a mistake, it will 799not forgive it, but will cruelly purge all the addresses. 800 801\paragraph{Statistics:} With the \verb|-statistics| option, the command 802becomes verbose. It prints out the number of deleted addresses and the number 803of rounds made to flush the address list. If this option is given 804twice, \verb|ip addr flush| also dumps all the deleted addresses 805in the format described in the previous subsection. 806 807\paragraph{Example:} Delete all the addresses from the private network 80810.0.0.0/8: 809\begin{verbatim} 810netadm@amber:~ # ip -s -s a f to 10/8 8112: dummy inet 10.7.7.7/16 brd 10.7.255.255 scope global dummy 8123: eth0 inet 10.10.7.7/16 brd 10.10.255.255 scope global eth0 8134: eth1 inet 10.8.7.7/16 brd 10.8.255.255 scope global eth1 814 815*** Round 1, deleting 3 addresses *** 816*** Flush is complete after 1 round *** 817netadm@amber:~ # 818\end{verbatim} 819Another instructive example is disabling IP on all the Ethernets: 820\begin{verbatim} 821netadm@amber:~ # ip -4 addr flush label "eth*" 822\end{verbatim} 823And the last example shows how to flush all the IPv6 addresses 824acquired by the host from stateless address autoconfiguration 825after you enabled forwarding or disabled autoconfiguration. 826\begin{verbatim} 827netadm@amber:~ # ip -6 addr flush dynamic 828\end{verbatim} 829 830 831 832\section{{\tt ip neighbour} --- neighbour/arp tables management} 833 834\paragraph{Abbreviations:} \verb|neighbour|, \verb|neighbor|, \verb|neigh|, 835\verb|n|. 836 837\paragraph{Object:} \verb|neighbour| objects establish bindings between protocol 838addresses and link layer addresses for hosts sharing the same link. 839Neighbour entries are organized into tables. The IPv4 neighbour table 840is known by another name --- the ARP table. 841 842The corresponding commands display neighbour bindings 843and their properties, add new neighbour entries and delete old ones. 844 845\paragraph{Commands:} \verb|add|, \verb|change|, \verb|replace|, 846\verb|delete|, \verb|flush| and \verb|show| (or \verb|list|). 847 848\paragraph{See also:} Appendix~\ref{PROXY-NEIGH}, p.\pageref{PROXY-NEIGH} 849describes how to manage proxy ARP/NDISC with the \verb|ip| utility. 850 851 852\subsection{{\tt ip neighbour add} --- add a new neighbour entry\\ 853 {\tt ip neighbour change} --- change an existing entry\\ 854 {\tt ip neighbour replace} --- add a new entry or change an existing one} 855 856\paragraph{Abbreviations:} \verb|add|, \verb|a|; \verb|change|, \verb|chg|; 857\verb|replace|, \verb|repl|. 858 859\paragraph{Description:} These commands create new neighbour records 860or update existing ones. 861 862\paragraph{Arguments:} 863 864\begin{itemize} 865\item \verb|to ADDRESS| (default) 866 867--- the protocol address of the neighbour. It is either an IPv4 or IPv6 address. 868 869\item \verb|dev NAME| 870 871--- the interface to which this neighbour is attached. 872 873 874\item \verb|lladdr LLADDRESS| 875 876--- the link layer address of the neighbour. \verb|LLADDRESS| can also be 877\verb|null|. 878 879\item \verb|nud NUD_STATE| 880 881--- the state of the neighbour entry. \verb|nud| is an abbreviation for ``Neighbour 882Unreachability Detection''. The state can take one of the following values: 883 884\begin{enumerate} 885\item \verb|permanent| --- the neighbour entry is valid forever and can be only be removed 886administratively. 887\item \verb|noarp| --- the neighbour entry is valid. No attempts to validate 888this entry will be made but it can be removed when its lifetime expires. 889\item \verb|reachable| --- the neighbour entry is valid until the reachability 890timeout expires. 891\item \verb|stale| --- the neighbour entry is valid but suspicious. 892This option to \verb|ip neigh| does not change the neighbour state if 893it was valid and the address is not changed by this command. 894\end{enumerate} 895 896\end{itemize} 897 898\paragraph{Examples:} 899\begin{itemize} 900\item \verb|ip neigh add 10.0.0.3 lladdr 0:0:0:0:0:1 dev eth0 nud perm| 901 902--- add a permanent ARP entry for the neighbour 10.0.0.3 on the device \verb|eth0|. 903 904\item \verb|ip neigh chg 10.0.0.3 dev eth0 nud reachable| 905 906--- change its state to \verb|reachable|. 907\end{itemize} 908 909 910\subsection{{\tt ip neighbour delete} --- delete a neighbour entry} 911 912\paragraph{Abbreviations:} \verb|delete|, \verb|del|, \verb|d|. 913 914\paragraph{Description:} This command invalidates a neighbour entry. 915 916\paragraph{Arguments:} The arguments are the same as with \verb|ip neigh add|, 917except that \verb|lladdr| and \verb|nud| are ignored. 918 919 920\paragraph{Example:} 921\begin{itemize} 922\item \verb|ip neigh del 10.0.0.3 dev eth0| 923 924--- invalidate an ARP entry for the neighbour 10.0.0.3 on the device \verb|eth0|. 925 926\end{itemize} 927 928\begin{NB} 929 The deleted neighbour entry will not disappear from the tables 930 immediately. If it is in use it cannot be deleted until the last 931 client releases it. Otherwise it will be destroyed during 932 the next garbage collection. 933\end{NB} 934 935 936\paragraph{Warning:} Attempts to delete or manually change 937a \verb|noarp| entry created by the kernel may result in unpredictable behaviour. 938Particularly, the kernel may try to resolve this address even 939on a \verb|NOARP| interface or if the address is multicast or broadcast. 940 941 942\subsection{{\tt ip neighbour show} --- list neighbour entries} 943 944\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|. 945 946\paragraph{Description:}This commands displays neighbour tables. 947 948\paragraph{Arguments:} 949 950\begin{itemize} 951 952\item \verb|to ADDRESS| (default) 953 954--- the prefix selecting the neighbours to list. 955 956\item \verb|dev NAME| 957 958--- only list the neighbours attached to this device. 959 960\item \verb|unused| 961 962--- only list neighbours which are not currently in use. 963 964\item \verb|nud NUD_STATE| 965 966--- only list neighbour entries in this state. \verb|NUD_STATE| takes 967values listed below or the special value \verb|all| which means all states. 968This option may occur more than once. If this option is absent, \verb|ip| 969lists all entries except for \verb|none| and \verb|noarp|. 970 971\end{itemize} 972 973 974\paragraph{Output format:} 975 976\begin{verbatim} 977kuznet@alisa:~ $ ip neigh ls 978:: dev lo lladdr 00:00:00:00:00:00 nud noarp 979fe80::200:cff:fe76:3f85 dev eth0 lladdr 00:00:0c:76:3f:85 router \ 980 nud stale 9810.0.0.0 dev lo lladdr 00:00:00:00:00:00 nud noarp 982193.233.7.254 dev eth0 lladdr 00:00:0c:76:3f:85 nud reachable 983193.233.7.85 dev eth0 lladdr 00:e0:1e:63:39:00 nud stale 984kuznet@alisa:~ $ 985\end{verbatim} 986 987The first word of each line is the protocol address of the neighbour. 988Then the device name follows. The rest of the line describes the contents of 989the neighbour entry identified by the pair (device, address). 990 991\verb|lladdr| is the link layer address of the neighbour. 992 993\verb|nud| is the state of the ``neighbour unreachability detection'' machine 994for this entry. The detailed description of the neighbour 995state machine can be found in~\cite{RFC-NDISC}. Here is the full list 996of the states with short descriptions: 997 998\begin{enumerate} 999\item\verb|none| --- the state of the neighbour is void. 1000\item\verb|incomplete| --- the neighbour is in the process of resolution. 1001\item\verb|reachable| --- the neighbour is valid and apparently reachable. 1002\item\verb|stale| --- the neighbour is valid, but is probably already 1003unreachable, so the kernel will try to check it at the first transmission. 1004\item\verb|delay| --- a packet has been sent to the stale neighbour and the kernel is waiting 1005for confirmation. 1006\item\verb|probe| --- the delay timer expired but no confirmation was received. 1007The kernel has started to probe the neighbour with ARP/NDISC messages. 1008\item\verb|failed| --- resolution has failed. 1009\item\verb|noarp| --- the neighbour is valid. No attempts to check the entry 1010will be made. 1011\item\verb|permanent| --- it is a \verb|noarp| entry, but only the administrator 1012may remove the entry from the neighbour table. 1013\end{enumerate} 1014 1015The link layer address is valid in all states except for \verb|none|, 1016\verb|failed| and \verb|incomplete|. 1017 1018IPv6 neighbours can be marked with the additional flag \verb|router| 1019which means that the neighbour introduced itself as an IPv6 router~\cite{RFC-NDISC}. 1020 1021\paragraph{Statistics:} The \verb|-statistics| option displays some usage 1022statistics, f.e.\ 1023 1024\begin{verbatim} 1025kuznet@alisa:~ $ ip -s n ls 193.233.7.254 1026193.233.7.254 dev eth0 lladdr 00:00:0c:76:3f:85 ref 5 used 12/13/20 \ 1027 nud reachable 1028kuznet@alisa:~ $ 1029\end{verbatim} 1030 1031Here \verb|ref| is the number of users of this entry 1032and \verb|used| is a triplet of time intervals in seconds 1033separated by slashes. In this case they show that: 1034 1035\begin{enumerate} 1036\item the entry was used 12 seconds ago. 1037\item the entry was confirmed 13 seconds ago. 1038\item the entry was updated 20 seconds ago. 1039\end{enumerate} 1040 1041\subsection{{\tt ip neighbour flush} --- flush neighbour entries} 1042 1043\paragraph{Abbreviations:} \verb|flush|, \verb|f|. 1044 1045\paragraph{Description:}This command flushes neighbour tables, selecting 1046entries to flush by some criteria. 1047 1048\paragraph{Arguments:} This command has the same arguments as \verb|show|. 1049The differences are that it does not run when no arguments are given, 1050and that the default neighbour states to be flushed do not include 1051\verb|permanent| and \verb|noarp|. 1052 1053 1054\paragraph{Statistics:} With the \verb|-statistics| option, the command 1055becomes verbose. It prints out the number of deleted neighbours and the number 1056of rounds made to flush the neighbour table. If the option is given 1057twice, \verb|ip neigh flush| also dumps all the deleted neighbours 1058in the format described in the previous subsection. 1059 1060\paragraph{Example:} 1061\begin{verbatim} 1062netadm@alisa:~ # ip -s -s n f 193.233.7.254 1063193.233.7.254 dev eth0 lladdr 00:00:0c:76:3f:85 ref 5 used 12/13/20 \ 1064 nud reachable 1065 1066*** Round 1, deleting 1 entries *** 1067*** Flush is complete after 1 round *** 1068netadm@alisa:~ # 1069\end{verbatim} 1070 1071 1072\section{{\tt ip route} --- routing table management} 1073\label{IP-ROUTE} 1074 1075\paragraph{Abbreviations:} \verb|route|, \verb|ro|, \verb|r|. 1076 1077\paragraph{Object:} \verb|route| entries in the kernel routing tables keep 1078information about paths to other networked nodes. 1079 1080Each route entry has a {\em key\/} consisting of a {\em prefix\/} 1081(i.e.\ a pair containing a network address and the length of its mask) and, 1082optionally, the TOS value. An IP packet matches the route if the highest 1083bits of its destination address are equal to the route prefix at least 1084up to the prefix length and if the TOS of the route is zero or equal to 1085the TOS of the packet. 1086 1087If several routes match the packet, the following pruning rules 1088are used to select the best one (see~\cite{RFC1812}): 1089\begin{enumerate} 1090\item The longest matching prefix is selected. All shorter ones 1091are dropped. 1092 1093\item If the TOS of some route with the longest prefix is equal to the TOS 1094of the packet, the routes with different TOS are dropped. 1095 1096If no exact TOS match was found and routes with TOS=0 exist, 1097the rest of routes are pruned. 1098 1099Otherwise, the route lookup fails. 1100 1101\item If several routes remain after the previous steps, then 1102the routes with the best preference values are selected. 1103 1104\item If we still have several routes, then the {\em first\/} of them 1105is selected. 1106 1107\begin{NB} 1108 Note the ambiguity of the last step. Unfortunately, Linux 1109 historically allows such a bizarre situation. The sense of the 1110word ``first'' depends on the order of route additions and it is practically 1111impossible to maintain a bundle of such routes in this order. 1112\end{NB} 1113 1114For simplicity we will limit ourselves to the case where such a situation 1115is impossible and routes are uniquely identified by the triplet 1116\{prefix, tos, preference\}. Actually, it is impossible to create 1117non-unique routes with \verb|ip| commands described in this section. 1118 1119One useful exception to this rule is the default route on non-forwarding 1120hosts. It is ``officially'' allowed to have several fallback routes 1121when several routers are present on directly connected networks. 1122In this case, Linux-2.2 makes ``dead gateway detection''~\cite{RFC1122} 1123controlled by neighbour unreachability detection and by advice 1124from transport protocols to select a working router, so the order 1125of the routes is not essential. However, in this case, 1126fiddling with default routes manually is not recommended. Use the Router Discovery 1127protocol (see Appendix~\ref{EXAMPLE-SETUP}, p.\pageref{EXAMPLE-SETUP}) 1128instead. Actually, Linux-2.2 IPv6 does not give user level applications 1129any access to default routes. 1130\end{enumerate} 1131 1132Certainly, the steps above are not performed exactly 1133in this sequence. Instead, the routing table in the kernel is kept 1134in some data structure to achieve the final result 1135with minimal cost. However, not depending on a particular 1136routing algorithm implemented in the kernel, we can summarize 1137the statements above as: a route is identified by the triplet 1138\{prefix, tos, preference\}. This {\em key\/} lets us locate 1139the route in the routing table. 1140 1141\paragraph{Route attributes:} Each route key refers to a routing 1142information record containing 1143the data required to deliver IP packets (f.e.\ output device and 1144next hop router) and some optional attributes (f.e. the path MTU or 1145the preferred source address when communicating with this destination). 1146These attributes are described in the following subsection. 1147 1148\paragraph{Route types:} \label{IP-ROUTE-TYPES} 1149It is important that the set 1150of required and optional attributes depend on the route {\em type\/}. 1151The most important route type 1152is \verb|unicast|. It describes real paths to other hosts. 1153As a rule, common routing tables contain only such routes. However, 1154there are other types of routes with different semantics. The 1155full list of types understood by Linux-2.2 is: 1156\begin{itemize} 1157\item \verb|unicast| --- the route entry describes real paths to the 1158destinations covered by the route prefix. 1159\item \verb|unreachable| --- these destinations are unreachable. Packets 1160are discarded and the ICMP message {\em host unreachable\/} is generated. 1161The local senders get an \verb|EHOSTUNREACH| error. 1162\item \verb|blackhole| --- these destinations are unreachable. Packets 1163are discarded silently. The local senders get an \verb|EINVAL| error. 1164\item \verb|prohibit| --- these destinations are unreachable. Packets 1165are discarded and the ICMP message {\em communication administratively 1166prohibited\/} is generated. The local senders get an \verb|EACCES| error. 1167\item \verb|local| --- the destinations are assigned to this 1168host. The packets are looped back and delivered locally. 1169\item \verb|broadcast| --- the destinations are broadcast addresses. 1170The packets are sent as link broadcasts. 1171\item \verb|throw| --- a special control route used together with policy 1172rules (see sec.\ref{IP-RULE}, p.\pageref{IP-RULE}). If such a route is selected, lookup 1173in this table is terminated pretending that no route was found. 1174Without policy routing it is equivalent to the absence of the route in the routing 1175table. The packets are dropped and the ICMP message {\em net unreachable\/} 1176is generated. The local senders get an \verb|ENETUNREACH| error. 1177\item \verb|nat| --- a special NAT route. Destinations covered by the prefix 1178are considered to be dummy (or external) addresses which require translation 1179to real (or internal) ones before forwarding. The addresses to translate to 1180are selected with the attribute \verb|via|. More about NAT is 1181in Appendix~\ref{ROUTE-NAT}, p.\pageref{ROUTE-NAT}. 1182\item \verb|anycast| --- ({\em not implemented\/}) the destinations are 1183{\em anycast\/} addresses assigned to this host. They are mainly equivalent 1184to \verb|local| with one difference: such addresses are invalid when used 1185as the source address of any packet. 1186\item \verb|multicast| --- a special type used for multicast routing. 1187It is not present in normal routing tables. 1188\end{itemize} 1189 1190\paragraph{Route tables:} Linux-2.2 can pack routes into several routing 1191tables identified by a number in the range from 1 to 255 or by 1192name from the file \verb|/etc/iproute2/rt_tables|. By default all normal 1193routes are inserted into the \verb|main| table (ID 254) and the kernel only uses 1194this table when calculating routes. 1195 1196Actually, one other table always exists, which is invisible but 1197even more important. It is the \verb|local| table (ID 255). This table 1198consists of routes for local and broadcast addresses. The kernel maintains 1199this table automatically and the administrator usually need not modify it 1200or even look at it. 1201 1202The multiple routing tables enter the game when {\em policy routing\/} 1203is used. See sec.\ref{IP-RULE}, p.\pageref{IP-RULE}. 1204In this case, the table identifier effectively becomes 1205one more parameter, which should be added to the triplet 1206\{prefix, tos, preference\} to uniquely identify the route. 1207 1208 1209\subsection{{\tt ip route add} --- add a new route\\ 1210 {\tt ip route change} --- change a route\\ 1211 {\tt ip route replace} --- change a route or add a new one} 1212\label{IP-ROUTE-ADD} 1213 1214\paragraph{Abbreviations:} \verb|add|, \verb|a|; \verb|change|, \verb|chg|; 1215 \verb|replace|, \verb|repl|. 1216 1217 1218\paragraph{Arguments:} 1219\begin{itemize} 1220\item \verb|to PREFIX| or \verb|to TYPE PREFIX| (default) 1221 1222--- the destination prefix of the route. If \verb|TYPE| is omitted, 1223\verb|ip| assumes type \verb|unicast|. Other values of \verb|TYPE| 1224are listed above. \verb|PREFIX| is an IP or IPv6 address optionally followed 1225by a slash and the prefix length. If the length of the prefix is missing, 1226\verb|ip| assumes a full-length host route. There is also a special 1227\verb|PREFIX| --- \verb|default| --- which is equivalent to IP \verb|0/0| or 1228to IPv6 \verb|::/0|. 1229 1230\item \verb|tos TOS| or \verb|dsfield TOS| 1231 1232--- the Type Of Service (TOS) key. This key has no associated mask and 1233the longest match is understood as: First, compare the TOS 1234of the route and of the packet. If they are not equal, then the packet 1235may still match a route with a zero TOS. \verb|TOS| is either an 8 bit hexadecimal 1236number or an identifier from {\tt /etc/iproute2/rt\_dsfield}. 1237 1238 1239\item \verb|metric NUMBER| or \verb|preference NUMBER| 1240 1241--- the preference value of the route. \verb|NUMBER| is an arbitrary 32bit number. 1242 1243\item \verb|table TABLEID| 1244 1245--- the table to add this route to. 1246\verb|TABLEID| may be a number or a string from the file 1247\verb|/etc/iproute2/rt_tables|. If this parameter is omitted, 1248\verb|ip| assumes the \verb|main| table, with the exception of 1249\verb|local|, \verb|broadcast| and \verb|nat| routes, which are 1250put into the \verb|local| table by default. 1251 1252\item \verb|dev NAME| 1253 1254--- the output device name. 1255 1256\item \verb|via ADDRESS| 1257 1258--- the address of the nexthop router. Actually, the sense of this field depends 1259on the route type. For normal \verb|unicast| routes it is either the true nexthop 1260router or, if it is a direct route installed in BSD compatibility mode, 1261it can be a local address of the interface. 1262For NAT routes it is the first address of the block of translated IP destinations. 1263 1264\item \verb|src ADDRESS| 1265 1266--- the source address to prefer when sending to the destinations 1267covered by the route prefix. 1268 1269\item \verb|realm REALMID| 1270 1271--- the realm to which this route is assigned. 1272\verb|REALMID| may be a number or a string from the file 1273\verb|/etc/iproute2/rt_realms|. Sec.\ref{RT-REALMS} (p.\pageref{RT-REALMS}) 1274contains more information on realms. 1275 1276\item \verb|mtu MTU| or \verb|mtu lock MTU| 1277 1278--- the MTU along the path to the destination. If the modifier \verb|lock| is 1279not used, the MTU may be updated by the kernel due to Path MTU Discovery. 1280If the modifier \verb|lock| is used, no path MTU discovery will be tried, 1281all packets will be sent without the DF bit in IPv4 case 1282or fragmented to MTU for IPv6. 1283 1284\item \verb|window NUMBER| 1285 1286--- the maximal window for TCP to advertise to these destinations, 1287measured in bytes. It limits maximal data bursts that our TCP 1288peers are allowed to send to us. 1289 1290\item \verb|rtt NUMBER| 1291 1292--- the initial RTT (``Round Trip Time'') estimate. 1293 1294 1295\item \verb|rttvar NUMBER| 1296 1297--- \threeonly the initial RTT variance estimate. 1298 1299 1300\item \verb|ssthresh NUMBER| 1301 1302--- \threeonly an estimate for the initial slow start threshold. 1303 1304 1305\item \verb|cwnd NUMBER| 1306 1307--- \threeonly the clamp for congestion window. It is ignored if the \verb|lock| 1308 flag is not used. 1309 1310 1311\item \verb|advmss NUMBER| 1312 1313--- \threeonly the MSS (``Maximal Segment Size'') to advertise to these 1314 destinations when establishing TCP connections. If it is not given, 1315 Linux uses a default value calculated from the first hop device MTU. 1316 1317\begin{NB} 1318 If the path to these destination is asymmetric, this guess may be wrong. 1319\end{NB} 1320 1321\item \verb|reordering NUMBER| 1322 1323--- \threeonly Maximal reordering on the path to this destination. 1324 If it is not given, Linux uses the value selected with \verb|sysctl| 1325 variable \verb|net/ipv4/tcp_reordering|. 1326 1327 1328 1329\item \verb|nexthop NEXTHOP| 1330 1331--- the nexthop of a multipath route. \verb|NEXTHOP| is a complex value 1332with its own syntax similar to the top level argument lists: 1333\begin{itemize} 1334\item \verb|via ADDRESS| is the nexthop router. 1335\item \verb|dev NAME| is the output device. 1336\item \verb|weight NUMBER| is a weight for this element of a multipath 1337route reflecting its relative bandwidth or quality. 1338\end{itemize} 1339 1340\item \verb|scope SCOPE_VAL| 1341 1342--- the scope of the destinations covered by the route prefix. 1343\verb|SCOPE_VAL| may be a number or a string from the file 1344\verb|/etc/iproute2/rt_scopes|. 1345If this parameter is omitted, 1346\verb|ip| assumes scope \verb|global| for all gatewayed \verb|unicast| 1347routes, scope \verb|link| for direct \verb|unicast| and \verb|broadcast| routes 1348and scope \verb|host| for \verb|local| routes. 1349 1350\item \verb|protocol RTPROTO| 1351 1352--- the routing protocol identifier of this route. 1353\verb|RTPROTO| may be a number or a string from the file 1354\verb|/etc/iproute2/rt_protos|. If the routing protocol ID is 1355not given, \verb|ip| assumes protocol \verb|boot| (i.e.\ 1356it assumes the route was added by someone who doesn't 1357understand what they are doing). Several protocol values have a fixed interpretation. 1358Namely: 1359\begin{itemize} 1360\item \verb|redirect| --- the route was installed due to an ICMP redirect. 1361\item \verb|kernel| --- the route was installed by the kernel during 1362autoconfiguration. 1363\item \verb|boot| --- the route was installed during the bootup sequence. 1364If a routing daemon starts, it will purge all of them. 1365\item \verb|static| --- the route was installed by the administrator 1366to override dynamic routing. Routing daemon will respect them 1367and, probably, even advertise them to its peers. 1368\item \verb|ra| --- the route was installed by Router Discovery protocol. 1369\end{itemize} 1370The rest of the values are not reserved and the administrator is free 1371to assign (or not to assign) protocol tags. At least, routing 1372daemons should take care of setting some unique protocol values, 1373f.e.\ as they are assigned in \verb|rtnetlink.h| or in \verb|rt_protos| 1374database. 1375 1376 1377\item \verb|onlink| 1378 1379--- pretend that the nexthop is directly attached to this link, 1380even if it does not match any interface prefix. One application of this 1381option may be found in~\cite{IP-TUNNELS}. 1382 1383\item \verb|equalize| 1384 1385--- allow packet by packet randomization on multipath routes. 1386Without this modifier, the route will be frozen to one selected 1387nexthop, so that load splitting will only occur on per-flow base. 1388\verb|equalize| only works if the kernel is patched. 1389 1390 1391\end{itemize} 1392 1393 1394\begin{NB} 1395 Actually there are more commands: \verb|prepend| does the same 1396 thing as classic \verb|route add|, i.e.\ adds a route, even if another 1397 route to the same destination exists. Its opposite case is \verb|append|, 1398 which adds the route to the end of the list. Avoid these 1399 features. 1400\end{NB} 1401\begin{NB} 1402 More sad news, IPv6 only understands the \verb|append| command correctly. 1403 All the others are translated into \verb|append| commands. Certainly, 1404 this will change in the future. 1405\end{NB} 1406 1407\paragraph{Examples:} 1408\begin{itemize} 1409\item add a plain route to network 10.0.0/24 via gateway 193.233.7.65 1410\begin{verbatim} 1411 ip route add 10.0.0/24 via 193.233.7.65 1412\end{verbatim} 1413\item change it to a direct route via the \verb|dummy| device 1414\begin{verbatim} 1415 ip ro chg 10.0.0/24 dev dummy 1416\end{verbatim} 1417\item add a default multipath route splitting the load between \verb|ppp0| 1418and \verb|ppp1| 1419\begin{verbatim} 1420 ip route add default scope global nexthop dev ppp0 \ 1421 nexthop dev ppp1 1422\end{verbatim} 1423Note the scope value. It is not necessary but it informs the kernel 1424that this route is gatewayed rather than direct. Actually, if you 1425know the addresses of remote endpoints it would be better to use the 1426\verb|via| parameter. 1427\item announce that the address 192.203.80.144 is not a real one, but 1428should be translated to 193.233.7.83 before forwarding 1429\begin{verbatim} 1430 ip route add nat 192.203.80.144 via 193.233.7.83 1431\end{verbatim} 1432Backward translation is setup with policy rules described 1433in the following section (sec.\ref{IP-RULE}, p.\pageref{IP-RULE}). 1434\end{itemize} 1435 1436\subsection{{\tt ip route delete} --- delete a route} 1437 1438\paragraph{Abbreviations:} \verb|delete|, \verb|del|, \verb|d|. 1439 1440\paragraph{Arguments:} \verb|ip route del| has the same arguments as 1441\verb|ip route add|, but their semantics are a bit different. 1442 1443Key values (\verb|to|, \verb|tos|, \verb|preference| and \verb|table|) 1444select the route to delete. If optional attributes are present, \verb|ip| 1445verifies that they coincide with the attributes of the route to delete. 1446If no route with the given key and attributes was found, \verb|ip route del| 1447fails. 1448\begin{NB} 1449Linux-2.0 had the option to delete a route selected only by prefix address, 1450ignoring its length (i.e.\ netmask). This option no longer exists 1451because it was ambiguous. However, look at {\tt ip route flush} 1452(sec.\ref{IP-ROUTE-FLUSH}, p.\pageref{IP-ROUTE-FLUSH}) which 1453provides similar and even richer functionality. 1454\end{NB} 1455 1456\paragraph{Example:} 1457\begin{itemize} 1458\item delete the multipath route created by the command in previous subsection 1459\begin{verbatim} 1460 ip route del default scope global nexthop dev ppp0 \ 1461 nexthop dev ppp1 1462\end{verbatim} 1463\end{itemize} 1464 1465 1466 1467\subsection{{\tt ip route show} --- list routes} 1468 1469\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|, \verb|l|. 1470 1471\paragraph{Description:} the command displays the contents of the routing tables 1472or the route(s) selected by some criteria. 1473 1474 1475\paragraph{Arguments:} 1476\begin{itemize} 1477\item \verb|to SELECTOR| (default) 1478 1479--- only select routes from the given range of destinations. \verb|SELECTOR| 1480consists of an optional modifier (\verb|root|, \verb|match| or \verb|exact|) 1481and a prefix. \verb|root PREFIX| selects routes with prefixes not shorter 1482than \verb|PREFIX|. F.e.\ \verb|root 0/0| selects the entire routing table. 1483\verb|match PREFIX| selects routes with prefixes not longer than 1484\verb|PREFIX|. F.e.\ \verb|match 10.0/16| selects \verb|10.0/16|, 1485\verb|10/8| and \verb|0/0|, but it does not select \verb|10.1/16| and 1486\verb|10.0.0/24|. And \verb|exact PREFIX| (or just \verb|PREFIX|) 1487selects routes with this exact prefix. If neither of these options 1488are present, \verb|ip| assumes \verb|root 0/0| i.e.\ it lists the entire table. 1489 1490 1491\item \verb|tos TOS| or \verb|dsfield TOS| 1492 1493 --- only select routes with the given TOS. 1494 1495 1496\item \verb|table TABLEID| 1497 1498 --- show the routes from this table(s). The default setting is to show 1499\verb|table| \verb|main|. \verb|TABLEID| may either be the ID of a real table 1500or one of the special values: 1501 \begin{itemize} 1502 \item \verb|all| --- list all of the tables. 1503 \item \verb|cache| --- dump the routing cache. 1504 \end{itemize} 1505\begin{NB} 1506 IPv6 has a single table. However, splitting it into \verb|main|, \verb|local| 1507 and \verb|cache| is emulated by the \verb|ip| utility. 1508\end{NB} 1509 1510\item \verb|cloned| or \verb|cached| 1511 1512--- list cloned routes i.e.\ routes which were dynamically forked from 1513other routes because some route attribute (f.e.\ MTU) was updated. 1514Actually, it is equivalent to \verb|table cache|. 1515 1516\item \verb|from SELECTOR| 1517 1518--- the same syntax as for \verb|to|, but it binds the source address range 1519rather than destinations. Note that the \verb|from| option only works with 1520cloned routes. 1521 1522\item \verb|protocol RTPROTO| 1523 1524--- only list routes of this protocol. 1525 1526 1527\item \verb|scope SCOPE_VAL| 1528 1529--- only list routes with this scope. 1530 1531\item \verb|type TYPE| 1532 1533--- only list routes of this type. 1534 1535\item \verb|dev NAME| 1536 1537--- only list routes going via this device. 1538 1539\item \verb|via PREFIX| 1540 1541--- only list routes going via the nexthop routers selected by \verb|PREFIX|. 1542 1543\item \verb|src PREFIX| 1544 1545--- only list routes with preferred source addresses selected 1546by \verb|PREFIX|. 1547 1548\item \verb|realm REALMID| or \verb|realms FROMREALM/TOREALM| 1549 1550--- only list routes with these realms. 1551 1552\end{itemize} 1553 1554\paragraph{Examples:} Let us count routes of protocol \verb|gated/bgp| 1555on a router: 1556\begin{verbatim} 1557kuznet@amber:~ $ ip ro ls proto gated/bgp | wc 1558 1413 9891 79010 1559kuznet@amber:~ $ 1560\end{verbatim} 1561To count the size of the routing cache, we have to use the \verb|-o| option 1562because cached attributes can take more than one line of output: 1563\begin{verbatim} 1564kuznet@amber:~ $ ip -o ro ls cloned | wc 1565 159 2543 18707 1566kuznet@amber:~ $ 1567\end{verbatim} 1568 1569 1570\paragraph{Output format:} The output of this command consists 1571of per route records separated by line feeds. 1572However, some records may consist 1573of more than one line: particularly, this is the case when the route 1574is cloned or you requested additional statistics. If the 1575\verb|-o| option was given, then line feeds separating lines inside 1576records are replaced with the backslash sign. 1577 1578The output has the same syntax as arguments given to {\tt ip route add}, 1579so that it can be understood easily. F.e.\ 1580\begin{verbatim} 1581kuznet@amber:~ $ ip ro ls 193.233.7/24 1582193.233.7.0/24 dev eth0 proto gated/conn scope link \ 1583 src 193.233.7.65 realms inr.ac 1584kuznet@amber:~ $ 1585\end{verbatim} 1586 1587If you list cloned entries, the output contains other attributes which 1588are evaluated during route calculation and updated during route 1589lifetime. An example of the output is: 1590\begin{verbatim} 1591kuznet@amber:~ $ ip ro ls 193.233.7.82 tab cache 1592193.233.7.82 from 193.233.7.82 dev eth0 src 193.233.7.65 \ 1593 realms inr.ac/inr.ac 1594 cache <src-direct,redirect> mtu 1500 rtt 300 iif eth0 1595193.233.7.82 dev eth0 src 193.233.7.65 realms inr.ac 1596 cache mtu 1500 rtt 300 1597kuznet@amber:~ $ 1598\end{verbatim} 1599\begin{NB} 1600 \label{NB-strange-route} 1601 The route looks a bit strange, doesn't it? Did you notice that 1602 it is a path from 193.233.7.82 back to 193.233.82? Well, you will 1603 see in the section on \verb|ip route get| (p.\pageref{NB-nature-of-strangeness}) 1604 how it appeared. 1605\end{NB} 1606The second line, starting with the word \verb|cache|, shows 1607additional attributes which normal routes do not possess. 1608Cached flags are summarized in angle brackets: 1609\begin{itemize} 1610\item \verb|local| --- packets are delivered locally. 1611It stands for loopback unicast routes, for broadcast routes 1612and for multicast routes, if this host is a member of the corresponding 1613group. 1614 1615\item \verb|reject| --- the path is bad. Any attempt to use it results 1616in an error. See attribute \verb|error| below (p.\pageref{IP-ROUTE-GET-error}). 1617 1618\item \verb|mc| --- the destination is multicast. 1619 1620\item \verb|brd| --- the destination is broadcast. 1621 1622\item \verb|src-direct| --- the source is on a directly connected 1623interface. 1624 1625\item \verb|redirected| --- the route was created by an ICMP Redirect. 1626 1627\item \verb|redirect| --- packets going via this route will 1628trigger an ICMP redirect. 1629 1630\item \verb|fastroute| --- the route is eligible to be used for fastroute. 1631 1632\item \verb|equalize| --- make packet by packet randomization 1633along this path. 1634 1635\item \verb|dst-nat| --- the destination address requires translation. 1636 1637\item \verb|src-nat| --- the source address requires translation. 1638 1639\item \verb|masq| --- the source address requires masquerading. 1640This feature disappeared in linux-2.4. 1641 1642\item \verb|notify| --- ({\em not implemented}) change/deletion 1643of this route will trigger RTNETLINK notification. 1644\end{itemize} 1645 1646Then some optional attributes follow: 1647\begin{itemize} 1648\item \verb|error| --- on \verb|reject| routes it is error code 1649returned to local senders when they try to use this route. 1650These error codes are translated into ICMP error codes, sent to remote 1651senders, according to the rules described above in the subsection 1652devoted to route types (p.\pageref{IP-ROUTE-TYPES}). 1653\label{IP-ROUTE-GET-error} 1654 1655\item \verb|expires| --- this entry will expire after this timeout. 1656 1657\item \verb|iif| --- the packets for this path are expected to arrive 1658on this interface. 1659\end{itemize} 1660 1661\paragraph{Statistics:} With the \verb|-statistics| option, more 1662information about this route is shown: 1663\begin{itemize} 1664\item \verb|users| --- the number of users of this entry. 1665\item \verb|age| --- shows when this route was last used. 1666\item \verb|used| --- the number of lookups of this route since its creation. 1667\end{itemize} 1668 1669 1670\subsection{{\tt ip route flush} --- flush routing tables} 1671\label{IP-ROUTE-FLUSH} 1672 1673\paragraph{Abbreviations:} \verb|flush|, \verb|f|. 1674 1675\paragraph{Description:} this command flushes routes selected 1676by some criteria. 1677 1678\paragraph{Arguments:} the arguments have the same syntax and semantics 1679as the arguments of \verb|ip route show|, but routing tables are not 1680listed but purged. The only difference is the default action: \verb|show| 1681dumps all the IP main routing table but \verb|flush| prints the helper page. 1682The reason for this difference does not require any explanation, does it? 1683 1684 1685\paragraph{Statistics:} With the \verb|-statistics| option, the command 1686becomes verbose. It prints out the number of deleted routes and the number 1687of rounds made to flush the routing table. If the option is given 1688twice, \verb|ip route flush| also dumps all the deleted routes 1689in the format described in the previous subsection. 1690 1691\paragraph{Examples:} The first example flushes all the 1692gatewayed routes from the main table (f.e.\ after a routing daemon crash). 1693\begin{verbatim} 1694netadm@amber:~ # ip -4 ro flush scope global type unicast 1695\end{verbatim} 1696This option deserves to be put into a scriptlet \verb|routef|. 1697\begin{NB} 1698This option was described in the \verb|route(8)| man page borrowed 1699from BSD, but was never implemented in Linux. 1700\end{NB} 1701 1702The second example flushes all IPv6 cloned routes: 1703\begin{verbatim} 1704netadm@amber:~ # ip -6 -s -s ro flush cache 17053ffe:2400::220:afff:fef4:c5d1 via 3ffe:2400::220:afff:fef4:c5d1 \ 1706 dev eth0 metric 0 1707 cache used 2 age 12sec mtu 1500 rtt 300 17083ffe:2400::280:adff:feb7:8034 via 3ffe:2400::280:adff:feb7:8034 \ 1709 dev eth0 metric 0 1710 cache used 2 age 15sec mtu 1500 rtt 300 17113ffe:2400::280:c8ff:fe59:5bcc via 3ffe:2400::280:c8ff:fe59:5bcc \ 1712 dev eth0 metric 0 1713 cache users 1 used 1 age 23sec mtu 1500 rtt 300 17143ffe:2400:0:1:2a0:ccff:fe66:1878 via 3ffe:2400:0:1:2a0:ccff:fe66:1878 \ 1715 dev eth1 metric 0 1716 cache used 2 age 20sec mtu 1500 rtt 300 17173ffe:2400:0:1:a00:20ff:fe71:fb30 via 3ffe:2400:0:1:a00:20ff:fe71:fb30 \ 1718 dev eth1 metric 0 1719 cache used 2 age 33sec mtu 1500 rtt 300 1720ff02::1 via ff02::1 dev eth1 metric 0 1721 cache users 1 used 1 age 45sec mtu 1500 rtt 300 1722 1723*** Round 1, deleting 6 entries *** 1724*** Flush is complete after 1 round *** 1725netadm@amber:~ # ip -6 -s -s ro flush cache 1726Nothing to flush. 1727netadm@amber:~ # 1728\end{verbatim} 1729 1730The third example flushes BGP routing tables after a \verb|gated| 1731death. 1732\begin{verbatim} 1733netadm@amber:~ # ip ro ls proto gated/bgp | wc 1734 1408 9856 78730 1735netadm@amber:~ # ip -s ro f proto gated/bgp 1736 1737*** Round 1, deleting 1408 entries *** 1738*** Flush is complete after 1 round *** 1739netadm@amber:~ # ip ro f proto gated/bgp 1740Nothing to flush. 1741netadm@amber:~ # ip ro ls proto gated/bgp 1742netadm@amber:~ # 1743\end{verbatim} 1744 1745 1746\subsection{{\tt ip route get} --- get a single route} 1747\label{IP-ROUTE-GET} 1748 1749\paragraph{Abbreviations:} \verb|get|, \verb|g|. 1750 1751\paragraph{Description:} this command gets a single route to a destination 1752and prints its contents exactly as the kernel sees it. 1753 1754\paragraph{Arguments:} 1755\begin{itemize} 1756\item \verb|to ADDRESS| (default) 1757 1758--- the destination address. 1759 1760\item \verb|from ADDRESS| 1761 1762--- the source address. 1763 1764\item \verb|tos TOS| or \verb|dsfield TOS| 1765 1766--- the Type Of Service. 1767 1768\item \verb|iif NAME| 1769 1770--- the device from which this packet is expected to arrive. 1771 1772\item \verb|oif NAME| 1773 1774--- force the output device on which this packet will be routed. 1775 1776\item \verb|connected| 1777 1778--- if no source address (option \verb|from|) was given, relookup 1779the route with the source set to the preferred address received from the first lookup. 1780If policy routing is used, it may be a different route. 1781 1782\end{itemize} 1783 1784Note that this operation is not equivalent to \verb|ip route show|. 1785\verb|show| shows existing routes. \verb|get| resolves them and 1786creates new clones if necessary. Essentially, \verb|get| 1787is equivalent to sending a packet along this path. 1788If the \verb|iif| argument is not given, the kernel creates a route 1789to output packets towards the requested destination. 1790This is equivalent to pinging the destination 1791with a subsequent {\tt ip route ls cache}, however, no packets are 1792actually sent. With the \verb|iif| argument, the kernel pretends 1793that a packet arrived from this interface and searches for 1794a path to forward the packet. 1795 1796\paragraph{Output format:} This command outputs routes in the same 1797format as \verb|ip route ls|. 1798 1799\paragraph{Examples:} 1800\begin{itemize} 1801\item Find a route to output packets to 193.233.7.82: 1802\begin{verbatim} 1803kuznet@amber:~ $ ip route get 193.233.7.82 1804193.233.7.82 dev eth0 src 193.233.7.65 realms inr.ac 1805 cache mtu 1500 rtt 300 1806kuznet@amber:~ $ 1807\end{verbatim} 1808 1809\item Find a route to forward packets arriving on \verb|eth0| 1810from 193.233.7.82 and destined for 193.233.7.82: 1811\begin{verbatim} 1812kuznet@amber:~ $ ip r g 193.233.7.82 from 193.233.7.82 iif eth0 1813193.233.7.82 from 193.233.7.82 dev eth0 src 193.233.7.65 \ 1814 realms inr.ac/inr.ac 1815 cache <src-direct,redirect> mtu 1500 rtt 300 iif eth0 1816kuznet@amber:~ $ 1817\end{verbatim} 1818\begin{NB} 1819 \label{NB-nature-of-strangeness} 1820 This is the command that created the funny route from 193.233.7.82 1821 looped back to 193.233.7.82 (cf.\ NB on~p.\pageref{NB-strange-route}). 1822 Note the \verb|redirect| flag on it. 1823\end{NB} 1824 1825\item Find a multicast route for packets arriving on \verb|eth0| 1826from host 193.233.7.82 and destined for multicast group 224.2.127.254 1827(it is assumed that a multicast routing daemon is running. 1828In this case, it is \verb|pimd|) 1829\begin{verbatim} 1830kuznet@amber:~ $ ip r g 224.2.127.254 from 193.233.7.82 iif eth0 1831multicast 224.2.127.254 from 193.233.7.82 dev lo \ 1832 src 193.233.7.65 realms inr.ac/cosmos 1833 cache <mc> iif eth0 Oifs: eth1 pimreg 1834kuznet@amber:~ $ 1835\end{verbatim} 1836This route differs from the ones seen before. It contains a ``normal'' part 1837and a ``multicast'' part. The normal part is used to deliver (or not to 1838deliver) the packet to local IP listeners. In this case the router 1839is not a member 1840of this group, so that route has no \verb|local| flag and only 1841forwards packets. The output device for such entries is always loopback. 1842The multicast part consists of an additional \verb|Oifs:| list showing 1843the output interfaces. 1844\end{itemize} 1845 1846 1847It is time for a more complicated example. Let us add an invalid 1848gatewayed route for a destination which is really directly connected: 1849\begin{verbatim} 1850netadm@alisa:~ # ip route add 193.233.7.98 via 193.233.7.254 1851netadm@alisa:~ # ip route get 193.233.7.98 1852193.233.7.98 via 193.233.7.254 dev eth0 src 193.233.7.90 1853 cache mtu 1500 rtt 3072 1854netadm@alisa:~ # 1855\end{verbatim} 1856and probe it with ping: 1857\begin{verbatim} 1858netadm@alisa:~ # ping -n 193.233.7.98 1859PING 193.233.7.98 (193.233.7.98) from 193.233.7.90 : 56 data bytes 1860From 193.233.7.254: Redirect Host(New nexthop: 193.233.7.98) 186164 bytes from 193.233.7.98: icmp_seq=0 ttl=255 time=3.5 ms 1862From 193.233.7.254: Redirect Host(New nexthop: 193.233.7.98) 186364 bytes from 193.233.7.98: icmp_seq=1 ttl=255 time=2.2 ms 186464 bytes from 193.233.7.98: icmp_seq=2 ttl=255 time=0.4 ms 186564 bytes from 193.233.7.98: icmp_seq=3 ttl=255 time=0.4 ms 186664 bytes from 193.233.7.98: icmp_seq=4 ttl=255 time=0.4 ms 1867^C 1868--- 193.233.7.98 ping statistics --- 18695 packets transmitted, 5 packets received, 0% packet loss 1870round-trip min/avg/max = 0.4/1.3/3.5 ms 1871netadm@alisa:~ # 1872\end{verbatim} 1873What happened? Router 193.233.7.254 understood that we have a much 1874better path to the destination and sent us an ICMP redirect message. 1875We may retry \verb|ip route get| to see what we have in the routing 1876tables now: 1877\begin{verbatim} 1878netadm@alisa:~ # ip route get 193.233.7.98 1879193.233.7.98 dev eth0 src 193.233.7.90 1880 cache <redirected> mtu 1500 rtt 3072 1881netadm@alisa:~ # 1882\end{verbatim} 1883 1884 1885 1886\section{{\tt ip rule} --- routing policy database management} 1887\label{IP-RULE} 1888 1889\paragraph{Abbreviations:} \verb|rule|, \verb|ru|. 1890 1891\paragraph{Object:} \verb|rule|s in the routing policy database control 1892the route selection algorithm. 1893 1894Classic routing algorithms used in the Internet make routing decisions 1895based only on the destination address of packets (and in theory, 1896but not in practice, on the TOS field). The seminal review of classic 1897routing algorithms and their modifications can be found in~\cite{RFC1812}. 1898 1899In some circumstances we want to route packets differently depending not only 1900on destination addresses, but also on other packet fields: source address, 1901IP protocol, transport protocol ports or even packet payload. 1902This task is called ``policy routing''. 1903 1904\begin{NB} 1905 ``policy routing'' $\neq$ ``routing policy''. 1906 1907\noindent ``policy routing'' $=$ ``cunning routing''. 1908 1909\noindent ``routing policy'' $=$ ``routing tactics'' or ``routing plan''. 1910\end{NB} 1911 1912To solve this task, the conventional destination based routing table, ordered 1913according to the longest match rule, is replaced with a ``routing policy 1914database'' (or RPDB), which selects routes 1915by executing some set of rules. The rules may have lots of keys of different 1916natures and therefore they have no natural ordering, but one imposed 1917by the administrator. Linux-2.2 RPDB is a linear list of rules 1918ordered by numeric priority value. 1919RPDB explicitly allows matching a few packet fields: 1920 1921\begin{itemize} 1922\item packet source address. 1923\item packet destination address. 1924\item TOS. 1925\item incoming interface (which is packet metadata, rather than a packet field). 1926\end{itemize} 1927 1928Matching IP protocols and transport ports is also possible, 1929indirectly, via \verb|ipchains|, by exploiting their ability 1930to mark some classes of packets with \verb|fwmark|. Therefore, 1931\verb|fwmark| is also included in the set of keys checked by rules. 1932 1933Each policy routing rule consists of a {\em selector\/} and an {\em action\/} 1934predicate. The RPDB is scanned in the order of increasing priority. The selector 1935of each rule is applied to \{source address, destination address, incoming 1936interface, tos, fwmark\} and, if the selector matches the packet, 1937the action is performed. The action predicate may return with success. 1938In this case, it will either give a route or failure indication 1939and the RPDB lookup is terminated. Otherwise, the RPDB program 1940continues on the next rule. 1941 1942What is the action, semantically? The natural action is to select the 1943nexthop and the output device. This is what 1944Cisco IOS~\cite{IOS} does. Let us call it ``match \& set''. 1945The Linux-2.2 approach is more flexible. The action includes 1946lookups in destination-based routing tables and selecting 1947a route from these tables according to the classic longest match algorithm. 1948The ``match \& set'' approach is the simplest case of the Linux one. It is realized 1949when a second level routing table contains a single default route. 1950Recall that Linux-2.2 supports multiple tables 1951managed with the \verb|ip route| command, described in the previous section. 1952 1953At startup time the kernel configures the default RPDB consisting of three 1954rules: 1955 1956\begin{enumerate} 1957\item Priority: 0, Selector: match anything, Action: lookup routing 1958table \verb|local| (ID 255). 1959The \verb|local| table is a special routing table containing 1960high priority control routes for local and broadcast addresses. 1961 1962Rule 0 is special. It cannot be deleted or overridden. 1963 1964 1965\item Priority: 32766, Selector: match anything, Action: lookup routing 1966table \verb|main| (ID 254). 1967The \verb|main| table is the normal routing table containing all non-policy 1968routes. This rule may be deleted and/or overridden with other 1969ones by the administrator. 1970 1971\item Priority: 32767, Selector: match anything, Action: lookup routing 1972table \verb|default| (ID 253). 1973The \verb|default| table is empty. It is reserved for some 1974post-processing if no previous default rules selected the packet. 1975This rule may also be deleted. 1976 1977\end{enumerate} 1978 1979Do not confuse routing tables with rules: rules point to routing tables, 1980several rules may refer to one routing table and some routing tables 1981may have no rules pointing to them. If the administrator deletes all the rules 1982referring to a table, the table is not used, but it still exists 1983and will disappear only after all the routes contained in it are deleted. 1984 1985 1986\paragraph{Rule attributes:} Each RPDB entry has additional 1987attributes. F.e.\ each rule has a pointer to some routing 1988table. NAT and masquerading rules have an attribute to select new IP 1989address to translate/masquerade. Besides that, rules have some 1990optional attributes, which routes have, namely \verb|realms|. 1991These values do not override those contained in the routing tables. They 1992are only used if the route did not select any attributes. 1993 1994 1995\paragraph{Rule types:} The RPDB may contain rules of the following 1996types: 1997\begin{itemize} 1998\item \verb|unicast| --- the rule prescribes to return the route found 1999in the routing table referenced by the rule. 2000\item \verb|blackhole| --- the rule prescribes to silently drop the packet. 2001\item \verb|unreachable| --- the rule prescribes to generate a ``Network 2002is unreachable'' error. 2003\item \verb|prohibit| --- the rule prescribes to generate 2004``Communication is administratively prohibited'' error. 2005\item \verb|nat| --- the rule prescribes to translate the source address 2006of the IP packet into some other value. More about NAT is 2007in Appendix~\ref{ROUTE-NAT}, p.\pageref{ROUTE-NAT}. 2008\end{itemize} 2009 2010 2011\paragraph{Commands:} \verb|add|, \verb|delete| and \verb|show| 2012(or \verb|list|). 2013 2014\subsection{{\tt ip rule add} --- insert a new rule\\ 2015 {\tt ip rule delete} --- delete a rule} 2016\label{IP-RULE-ADD} 2017 2018\paragraph{Abbreviations:} \verb|add|, \verb|a|; \verb|delete|, \verb|del|, 2019 \verb|d|. 2020 2021\paragraph{Arguments:} 2022 2023\begin{itemize} 2024\item \verb|type TYPE| (default) 2025 2026--- the type of this rule. The list of valid types was given in the previous 2027subsection. 2028 2029\item \verb|from PREFIX| 2030 2031--- select the source prefix to match. 2032 2033\item \verb|to PREFIX| 2034 2035--- select the destination prefix to match. 2036 2037\item \verb|iif NAME| 2038 2039--- select the incoming device to match. If the interface is loopback, 2040the rule only matches packets originating from this host. This means that you 2041may create separate routing tables for forwarded and local packets and, 2042hence, completely segregate them. 2043 2044\item \verb|tos TOS| or \verb|dsfield TOS| 2045 2046--- select the TOS value to match. 2047 2048\item \verb|fwmark MARK| 2049 2050--- select the \verb|fwmark| value to match. 2051 2052\item \verb|priority PREFERENCE| 2053 2054--- the priority of this rule. Each rule should have an explicitly 2055set {\em unique\/} priority value. 2056\begin{NB} 2057 Really, for historical reasons \verb|ip rule add| does not require a 2058 priority value and allows them to be non-unique. 2059 If the user does not supplied a priority, it is selected by the kernel. 2060 If the user creates a rule with a priority value that 2061 already exists, the kernel does not reject the request. It adds 2062 the new rule before all old rules of the same priority. 2063 2064 It is mistake in design, no more. And it will be fixed one day, 2065 so do not rely on this feature. Use explicit priorities. 2066\end{NB} 2067 2068 2069\item \verb|table TABLEID| 2070 2071--- the routing table identifier to lookup if the rule selector matches. 2072 2073\item \verb|realms FROM/TO| 2074 2075--- Realms to select if the rule matched and the routing table lookup 2076succeeded. Realm \verb|TO| is only used if the route did not select 2077any realm. 2078 2079\item \verb|nat ADDRESS| 2080 2081--- The base of the IP address block to translate (for source addresses). 2082The \verb|ADDRESS| may be either the start of the block of NAT addresses 2083(selected by NAT routes) or in linux-2.2 a local host address (or even zero). 2084In the last case the router does not translate the packets, 2085but masquerades them to this address; this feature disappered in 2.4. 2086More about NAT is in Appendix~\ref{ROUTE-NAT}, 2087p.\pageref{ROUTE-NAT}. 2088 2089\end{itemize} 2090 2091\paragraph{Warning:} Changes to the RPDB made with these commands 2092do not become active immediately. It is assumed that after 2093a script finishes a batch of updates, it flushes the routing cache 2094with \verb|ip route flush cache|. 2095 2096\paragraph{Examples:} 2097\begin{itemize} 2098\item Route packets with source addresses from 192.203.80/24 2099according to routing table \verb|inr.ruhep|: 2100\begin{verbatim} 2101ip ru add from 192.203.80.0/24 table inr.ruhep prio 220 2102\end{verbatim} 2103 2104\item Translate packet source address 193.233.7.83 into 192.203.80.144 2105and route it according to table \#1 (actually, it is \verb|inr.ruhep|): 2106\begin{verbatim} 2107ip ru add from 193.233.7.83 nat 192.203.80.144 table 1 prio 320 2108\end{verbatim} 2109 2110\item Delete the unused default rule: 2111\begin{verbatim} 2112ip ru del prio 32767 2113\end{verbatim} 2114 2115\end{itemize} 2116 2117 2118 2119\subsection{{\tt ip rule show} --- list rules} 2120\label{IP-RULE-SHOW} 2121 2122\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|, \verb|l|. 2123 2124 2125\paragraph{Arguments:} Good news, this is one command that has no arguments. 2126 2127\paragraph{Output format:} 2128 2129\begin{verbatim} 2130kuznet@amber:~ $ ip ru ls 21310: from all lookup local 2132200: from 192.203.80.0/24 to 193.233.7.0/24 lookup main 2133210: from 192.203.80.0/24 to 192.203.80.0/24 lookup main 2134220: from 192.203.80.0/24 lookup inr.ruhep realms inr.ruhep/radio-msu 2135300: from 193.233.7.83 to 193.233.7.0/24 lookup main 2136310: from 193.233.7.83 to 192.203.80.0/24 lookup main 2137320: from 193.233.7.83 lookup inr.ruhep map-to 192.203.80.144 213832766: from all lookup main 2139kuznet@amber:~ $ 2140\end{verbatim} 2141 2142In the first column is the rule priority value followed 2143by a colon. Then the selectors follow. Each key is prefixed 2144with the same keyword that was used to create the rule. 2145 2146The keyword \verb|lookup| is followed by a routing table identifier, 2147as it is recorded in the file \verb|/etc/iproute2/rt_tables|. 2148 2149If the rule does NAT (f.e.\ rule \#320), it is shown by the keyword 2150\verb|map-to| followed by the start of the block of addresses to map. 2151 2152The sense of this example is pretty simple. The prefixes 2153192.203.80.0/24 and 193.233.7.0/24 form the internal network, but 2154they are routed differently when the packets leave it. 2155Besides that, the host 193.233.7.83 is translated into 2156another prefix to look like 192.203.80.144 when talking 2157to the outer world. 2158 2159 2160 2161\section{{\tt ip maddress} --- multicast addresses management} 2162\label{IP-MADDR} 2163 2164\paragraph{Object:} \verb|maddress| objects are multicast addresses. 2165 2166\paragraph{Commands:} \verb|add|, \verb|delete|, \verb|show| (or \verb|list|). 2167 2168\subsection{{\tt ip maddress show} --- list multicast addresses} 2169 2170\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|, \verb|l|. 2171 2172\paragraph{Arguments:} 2173 2174\begin{itemize} 2175 2176\item \verb|dev NAME| (default) 2177 2178--- the device name. 2179 2180\end{itemize} 2181 2182\paragraph{Output format:} 2183 2184\begin{verbatim} 2185kuznet@alisa:~ $ ip maddr ls dummy 21862: dummy 2187 link 33:33:00:00:00:01 2188 link 01:00:5e:00:00:01 2189 inet 224.0.0.1 users 2 2190 inet6 ff02::1 2191kuznet@alisa:~ $ 2192\end{verbatim} 2193 2194The first line of the output shows the interface index and its name. 2195Then the multicast address list follows. Each line starts with the 2196protocol identifier. The word \verb|link| denotes a link layer 2197multicast addresses. 2198 2199If a multicast address has more than one user, the number 2200of users is shown after the \verb|users| keyword. 2201 2202One additional feature not present in the example above 2203is the \verb|static| flag, which indicates that the address was joined 2204with \verb|ip maddr add|. See the following subsection. 2205 2206 2207 2208\subsection{{\tt ip maddress add} --- add a multicast address\\ 2209 {\tt ip maddress delete} --- delete a multicast address} 2210 2211\paragraph{Abbreviations:} \verb|add|, \verb|a|; \verb|delete|, \verb|del|, \verb|d|. 2212 2213\paragraph{Description:} these commands attach/detach 2214a static link layer multicast address to listen on the interface. 2215Note that it is impossible to join protocol multicast groups 2216statically. This command only manages link layer addresses. 2217 2218 2219\paragraph{Arguments:} 2220 2221\begin{itemize} 2222\item \verb|address LLADDRESS| (default) 2223 2224--- the link layer multicast address. 2225 2226\item \verb|dev NAME| 2227 2228--- the device to join/leave this multicast address. 2229 2230\end{itemize} 2231 2232 2233\paragraph{Example:} Let us continue with the example from the previous subsection. 2234 2235\begin{verbatim} 2236netadm@alisa:~ # ip maddr add 33:33:00:00:00:01 dev dummy 2237netadm@alisa:~ # ip -0 maddr ls dummy 22382: dummy 2239 link 33:33:00:00:00:01 users 2 static 2240 link 01:00:5e:00:00:01 2241netadm@alisa:~ # ip maddr del 33:33:00:00:00:01 dev dummy 2242\end{verbatim} 2243 2244\begin{NB} 2245 Neither \verb|ip| nor the kernel check for multicast address validity. 2246 Particularly, this means that you can try to load a unicast address 2247 instead of a multicast address. Most drivers will ignore such addresses, 2248 but several (f.e.\ Tulip) will intern it to their on-board filter. 2249 The effects may be strange. Namely, the addresses become additional 2250 local link addresses and, if you loaded the address of another host 2251 to the router, wait for duplicated packets on the wire. 2252 It is not a bug, but rather a hole in the API and intra-kernel interfaces. 2253 This feature is really more useful for traffic monitoring, but using it 2254 with Linux-2.2 you {\em have to\/} be sure that the host is not 2255 a router and, especially, that it is not a transparent proxy or masquerading 2256 agent. 2257\end{NB} 2258 2259 2260 2261\section{{\tt ip mroute} --- multicast routing cache management} 2262\label{IP-MROUTE} 2263 2264\paragraph{Abbreviations:} \verb|mroute|, \verb|mr|. 2265 2266\paragraph{Object:} \verb|mroute| objects are multicast routing cache 2267entries created by a user level mrouting daemon 2268(f.e.\ \verb|pimd| or \verb|mrouted|). 2269 2270Due to the limitations of the current interface to the multicast routing 2271engine, it is impossible to change \verb|mroute| objects administratively, 2272so we may only display them. This limitation will be removed 2273in the future. 2274 2275\paragraph{Commands:} \verb|show| (or \verb|list|). 2276 2277 2278\subsection{{\tt ip mroute show} --- list mroute cache entries} 2279 2280\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|, \verb|l|. 2281 2282\paragraph{Arguments:} 2283 2284\begin{itemize} 2285\item \verb|to PREFIX| (default) 2286 2287--- the prefix selecting the destination multicast addresses to list. 2288 2289 2290\item \verb|iif NAME| 2291 2292--- the interface on which multicast packets are received. 2293 2294 2295\item \verb|from PREFIX| 2296 2297--- the prefix selecting the IP source addresses of the multicast route. 2298 2299 2300\end{itemize} 2301 2302\paragraph{Output format:} 2303 2304\begin{verbatim} 2305kuznet@amber:~ $ ip mroute ls 2306(193.232.127.6, 224.0.1.39) Iif: unresolved 2307(193.232.244.34, 224.0.1.40) Iif: unresolved 2308(193.233.7.65, 224.66.66.66) Iif: eth0 Oifs: pimreg 2309kuznet@amber:~ $ 2310\end{verbatim} 2311 2312Each line shows one (S,G) entry in the multicast routing cache, 2313where S is the source address and G is the multicast group. \verb|Iif| is 2314the interface on which multicast packets are expected to arrive. 2315If the word \verb|unresolved| is there instead of the interface name, 2316it means that the routing daemon still hasn't resolved this entry. 2317The keyword \verb|oifs| is followed by a list of output interfaces, separated 2318by spaces. If a multicast routing entry is created with non-trivial 2319TTL scope, administrative distances are appended to the device names 2320in the \verb|oifs| list. 2321 2322\paragraph{Statistics:} The \verb|-statistics| option also prints the 2323number of packets and bytes forwarded along this route and 2324the number of packets that arrived on the wrong interface, if this number is not zero. 2325 2326\begin{verbatim} 2327kuznet@amber:~ $ ip -s mr ls 224.66/16 2328(193.233.7.65, 224.66.66.66) Iif: eth0 Oifs: pimreg 2329 9383 packets, 300256 bytes 2330kuznet@amber:~ $ 2331\end{verbatim} 2332 2333 2334\section{{\tt ip tunnel} --- tunnel configuration} 2335\label{IP-TUNNEL} 2336 2337\paragraph{Abbreviations:} \verb|tunnel|, \verb|tunl|. 2338 2339\paragraph{Object:} \verb|tunnel| objects are tunnels, encapsulating 2340packets in IPv4 packets and then sending them over the IP infrastructure. 2341 2342\paragraph{Commands:} \verb|add|, \verb|delete|, \verb|change|, \verb|show| 2343(or \verb|list|). 2344 2345\paragraph{See also:} A more informal discussion of tunneling 2346over IP and the \verb|ip tunnel| command can be found in~\cite{IP-TUNNELS}. 2347 2348\subsection{{\tt ip tunnel add} --- add a new tunnel\\ 2349 {\tt ip tunnel change} --- change an existing tunnel\\ 2350 {\tt ip tunnel delete} --- destroy a tunnel} 2351 2352\paragraph{Abbreviations:} \verb|add|, \verb|a|; \verb|change|, \verb|chg|; 2353\verb|delete|, \verb|del|, \verb|d|. 2354 2355 2356\paragraph{Arguments:} 2357 2358\begin{itemize} 2359 2360\item \verb|name NAME| (default) 2361 2362--- select the tunnel device name. 2363 2364\item \verb|mode MODE| 2365 2366--- set the tunnel mode. Three modes are currently available: 2367 \verb|ipip|, \verb|sit| and \verb|gre|. 2368 2369\item \verb|remote ADDRESS| 2370 2371--- set the remote endpoint of the tunnel. 2372 2373\item \verb|local ADDRESS| 2374 2375--- set the fixed local address for tunneled packets. 2376It must be an address on another interface of this host. 2377 2378\item \verb|ttl N| 2379 2380--- set a fixed TTL \verb|N| on tunneled packets. 2381 \verb|N| is a number in the range 1--255. 0 is a special value 2382 meaning that packets inherit the TTL value. 2383 The default value is: \verb|inherit|. 2384 2385\item \verb|tos T| or \verb|dsfield T| 2386 2387--- set a fixed TOS \verb|T| on tunneled packets. 2388 The default value is: \verb|inherit|. 2389 2390 2391 2392\item \verb|dev NAME| 2393 2394--- bind the tunnel to the device \verb|NAME| so that 2395 tunneled packets will only be routed via this device and will 2396 not be able to escape to another device when the route to endpoint changes. 2397 2398\item \verb|nopmtudisc| 2399 2400--- disable Path MTU Discovery on this tunnel. 2401 It is enabled by default. Note that a fixed ttl is incompatible 2402 with this option: tunnelling with a fixed ttl always makes pmtu discovery. 2403 2404\item \verb|key K|, \verb|ikey K|, \verb|okey K| 2405 2406--- (only GRE tunnels) use keyed GRE with key \verb|K|. \verb|K| is 2407 either a number or an IP address-like dotted quad. 2408 The \verb|key| parameter sets the key to use in both directions. 2409 The \verb|ikey| and \verb|okey| parameters set different keys for input and output. 2410 2411 2412\item \verb|csum|, \verb|icsum|, \verb|ocsum| 2413 2414--- (only GRE tunnels) generate/require checksums for tunneled packets. 2415 The \verb|ocsum| flag calculates checksums for outgoing packets. 2416 The \verb|icsum| flag requires that all input packets have the correct 2417 checksum. The \verb|csum| flag is equivalent to the combination 2418 ``\verb|icsum| \verb|ocsum|''. 2419 2420\item \verb|seq|, \verb|iseq|, \verb|oseq| 2421 2422--- (only GRE tunnels) serialize packets. 2423 The \verb|oseq| flag enables sequencing of outgoing packets. 2424 The \verb|iseq| flag requires that all input packets are serialized. 2425 The \verb|seq| flag is equivalent to the combination ``\verb|iseq| \verb|oseq|''. 2426 2427\begin{NB} 2428 I think this option does not 2429 work. At least, I did not test it, did not debug it and 2430 do not even understand how it is supposed to work or for what 2431 purpose Cisco planned to use it. Do not use it. 2432\end{NB} 2433 2434 2435\end{itemize} 2436 2437\paragraph{Example:} Create a pointopoint IPv6 tunnel with maximal TTL of 32. 2438\begin{verbatim} 2439netadm@amber:~ # ip tunl add Cisco mode sit remote 192.31.7.104 \ 2440 local 192.203.80.142 ttl 32 2441\end{verbatim} 2442 2443\subsection{{\tt ip tunnel show} --- list tunnels} 2444 2445\paragraph{Abbreviations:} \verb|show|, \verb|list|, \verb|sh|, \verb|ls|, \verb|l|. 2446 2447 2448\paragraph{Arguments:} None. 2449 2450\paragraph{Output format:} 2451\begin{verbatim} 2452kuznet@amber:~ $ ip tunl ls Cisco 2453Cisco: ipv6/ip remote 192.31.7.104 local 192.203.80.142 ttl 32 2454kuznet@amber:~ $ 2455\end{verbatim} 2456The line starts with the tunnel device name followed by a colon. 2457Then the tunnel mode follows. The parameters of the tunnel are listed 2458with the same keywords that were used when creating the tunnel. 2459 2460\paragraph{Statistics:} 2461 2462\begin{verbatim} 2463kuznet@amber:~ $ ip -s tunl ls Cisco 2464Cisco: ipv6/ip remote 192.31.7.104 local 192.203.80.142 ttl 32 2465RX: Packets Bytes Errors CsumErrs OutOfSeq Mcasts 2466 12566 1707516 0 0 0 0 2467TX: Packets Bytes Errors DeadLoop NoRoute NoBufs 2468 13445 1879677 0 0 0 0 2469kuznet@amber:~ $ 2470\end{verbatim} 2471Essentially, these numbers are the same as the numbers 2472printed with {\tt ip -s link show} 2473(sec.\ref{IP-LINK-SHOW}, p.\pageref{IP-LINK-SHOW}) but the tags are different 2474to reflect that they are tunnel specific. 2475\begin{itemize} 2476\item \verb|CsumErrs| --- the total number of packets dropped 2477because of checksum failures for a GRE tunnel with checksumming enabled. 2478\item \verb|OutOfSeq| --- the total number of packets dropped 2479because they arrived out of sequence for a GRE tunnel with 2480serialization enabled. 2481\item \verb|Mcasts| --- the total number of multicast packets 2482received on a broadcast GRE tunnel. 2483\item \verb|DeadLoop| --- the total number of packets which were not 2484transmitted because the tunnel is looped back to itself. 2485\item \verb|NoRoute| --- the total number of packets which were not 2486transmitted because there is no IP route to the remote endpoint. 2487\item \verb|NoBufs| --- the total number of packets which were not 2488transmitted because the kernel failed to allocate a buffer. 2489\end{itemize} 2490 2491 2492\section{{\tt ip monitor} and {\tt rtmon} --- state monitoring} 2493\label{IP-MONITOR} 2494 2495The \verb|ip| utility can monitor the state of devices, addresses 2496and routes continuously. This option has a slightly different format. 2497Namely, 2498the \verb|monitor| command is the first in the command line and then 2499the object list follows: 2500\begin{verbatim} 2501 ip monitor [ file FILE ] [ all | OBJECT-LIST ] 2502\end{verbatim} 2503\verb|OBJECT-LIST| is the list of object types that we want to monitor. 2504It may contain \verb|link|, \verb|address| and \verb|route|. 2505If no \verb|file| argument is given, \verb|ip| opens RTNETLINK, 2506listens on it and dumps state changes in the format described 2507in previous sections. 2508 2509If a file name is given, it does not listen on RTNETLINK, 2510but opens the file containing RTNETLINK messages saved in binary format 2511and dumps them. Such a history file can be generated with the 2512\verb|rtmon| utility. This utility has a command line syntax similar to 2513\verb|ip monitor|. 2514Ideally, \verb|rtmon| should be started before 2515the first network configuration command is issued. F.e.\ if 2516you insert: 2517\begin{verbatim} 2518 rtmon file /var/log/rtmon.log 2519\end{verbatim} 2520in a startup script, you will be able to view the full history 2521later. 2522 2523Certainly, it is possible to start \verb|rtmon| at any time. 2524It prepends the history with the state snapshot dumped at the moment 2525of starting. 2526 2527 2528\section{Route realms and policy propagation, {\tt rtacct}} 2529\label{RT-REALMS} 2530 2531On routers using OSPF ASE or, especially, the BGP protocol, routing 2532tables may be huge. If we want to classify or to account for the packets 2533per route, we will have to keep lots of information. Even worse, if we 2534want to distinguish the packets not only by their destination, but 2535also by their source, the task gets quadratic complexity and its solution 2536is physically impossible. 2537 2538One approach to propagating the policy from routing protocols 2539to the forwarding engine has been proposed in~\cite{IOS-BGP-PP}. 2540Essentially, Cisco Policy Propagation via BGP is based on the fact 2541that dedicated routers all have the RIB (Routing Information Base) 2542close to the forwarding engine, so policy routing rules can 2543check all the route attributes, including ASPATH information 2544and community strings. 2545 2546The Linux architecture, splitting the RIB (maintained by a user level 2547daemon) and the kernel based FIB (Forwarding Information Base), 2548does not allow such a simple approach. 2549 2550It is to our fortune because there is another solution 2551which allows even more flexible policy and richer semantics. 2552 2553Namely, routes can be clustered together in user space, based on their 2554attributes. F.e.\ a BGP router knows route ASPATH, its community; 2555an OSPF router knows the route tag or its area. The administrator, when adding 2556routes manually, also knows their nature. Providing that the number of such 2557aggregates (we call them {\em realms\/}) is low, the task of full 2558classification both by source and destination becomes quite manageable. 2559 2560So each route may be assigned to a realm. It is assumed that 2561this identification is made by a routing daemon, but static routes 2562can also be handled manually with \verb|ip route| (see sec.\ref{IP-ROUTE}, 2563p.\pageref{IP-ROUTE}). 2564\begin{NB} 2565 There is a patch to \verb|gated|, allowing classification of routes 2566 to realms with all the set of policy rules implemented in \verb|gated|: 2567 by prefix, by ASPATH, by origin, by tag etc. 2568\end{NB} 2569 2570To facilitate the construction (f.e.\ in case the routing 2571daemon is not aware of realms), missing realms may be completed 2572with routing policy rules, see sec.~\ref{IP-RULE}, p.\pageref{IP-RULE}. 2573 2574For each packet the kernel calculates a tuple of realms: source realm 2575and destination realm, using the following algorithm: 2576 2577\begin{enumerate} 2578\item If the route has a realm, the destination realm of the packet is set to it. 2579\item If the rule has a source realm, the source realm of the packet is set to it. 2580If the destination realm was not inherited from the route and the rule has a destination realm, 2581it is also set. 2582\item If at least one of the realms is still unknown, the kernel finds 2583the reversed route to the source of the packet. 2584\item If the source realm is still unknown, get it from the reversed route. 2585\item If one of the realms is still unknown, swap the realms of reversed 2586routes and apply step 2 again. 2587\end{enumerate} 2588 2589After this procedure is completed we know what realm the packet 2590arrived from and the realm where it is going to propagate to. 2591If some of the realms are unknown, they are initialized to zero 2592(or realm \verb|unknown|). 2593 2594The main application of realms is the TC \verb|route| classifier~\cite{TC-CREF}, 2595where they are used to help assign packets to traffic classes, 2596to account, police and schedule them according to this 2597classification. 2598 2599A much simpler but still very useful application is incoming packet 2600accounting by realms. The kernel gathers a packet statistics summary 2601which can be viewed with the \verb|rtacct| utility. 2602\begin{verbatim} 2603kuznet@amber:~ $ rtacct russia 2604Realm BytesTo PktsTo BytesFrom PktsFrom 2605russia 20576778 169176 47080168 153805 2606kuznet@amber:~ $ 2607\end{verbatim} 2608This shows that this router received 153805 packets from 2609the realm \verb|russia| and forwarded 169176 packets to \verb|russia|. 2610The realm \verb|russia| consists of routes with ASPATHs not leaving 2611Russia. 2612 2613Note that locally originating packets are not accounted here, 2614\verb|rtacct| shows incoming packets only. Using the \verb|route| 2615classifier (see~\cite{TC-CREF}) you can get even more detailed 2616accounting information about outgoing packets, optionally 2617summarizing traffic not only by source or destination, but 2618by any pair of source and destination realms. 2619 2620 2621\begin{thebibliography}{99} 2622\addcontentsline{toc}{section}{References} 2623\bibitem{RFC-NDISC} T.~Narten, E.~Nordmark, W.~Simpson. 2624``Neighbor Discovery for IP Version 6 (IPv6)'', RFC-2461. 2625 2626\bibitem{RFC-ADDRCONF} S.~Thomson, T.~Narten. 2627``IPv6 Stateless Address Autoconfiguration'', RFC-2462. 2628 2629\bibitem{RFC1812} F.~Baker. 2630``Requirements for IP Version 4 Routers'', RFC-1812. 2631 2632\bibitem{RFC1122} R.~T.~Braden. 2633``Requirements for Internet hosts --- communication layers'', RFC-1122. 2634 2635\bibitem{IOS} ``Cisco IOS Release 12.0 Network Protocols 2636Command Reference, Part 1'' and 2637``Cisco IOS Release 12.0 Quality of Service Solutions 2638Configuration Guide: Configuring Policy-Based Routing'',\\ 2639http://www.cisco.com/univercd/cc/td/doc/product/software/ios120. 2640 2641\bibitem{IP-TUNNELS} A.~N.~Kuznetsov. 2642``Tunnels over IP in Linux-2.2'', \\ 2643In: {\tt ftp://ftp.inr.ac.ru/ip-routing/iproute2-current.tar.gz}. 2644 2645\bibitem{TC-CREF} A.~N.~Kuznetsov. ``TC Command Reference'',\\ 2646In: {\tt ftp://ftp.inr.ac.ru/ip-routing/iproute2-current.tar.gz}. 2647 2648\bibitem{IOS-BGP-PP} ``Cisco IOS Release 12.0 Quality of Service Solutions 2649Configuration Guide: Configuring QoS Policy Propagation via 2650Border Gateway Protocol'',\\ 2651http://www.cisco.com/univercd/cc/td/doc/product/software/ios120. 2652 2653\bibitem{RFC-DHCP} R.~Droms. 2654``Dynamic Host Configuration Protocol.'', RFC-2131 2655 2656\end{thebibliography} 2657 2658 2659 2660 2661\appendix 2662\addcontentsline{toc}{section}{Appendix} 2663 2664\section{Source address selection} 2665\label{ADDR-SEL} 2666 2667When a host creates an IP packet, it must select some source 2668address. Correct source address selection is a critical procedure, 2669because it gives the receiver the information needed to deliver a 2670reply. If the source is selected incorrectly, in the best case, 2671the backward path may appear different to the forward one which 2672is harmful for performance. In the worst case, when the addresses 2673are administratively scoped, the reply may be lost entirely. 2674 2675Linux-2.2 selects source addresses using the following algorithm: 2676 2677\begin{itemize} 2678\item 2679The application may select a source address explicitly with \verb|bind(2)| 2680syscall or supplying it to \verb|sendmsg(2)| via the ancillary data object 2681\verb|IP_PKTINFO|. In this case the kernel only checks the validity 2682of the address and never tries to ``improve'' an incorrect user choice, 2683generating an error instead. 2684\begin{NB} 2685 Never say ``Never''. The sysctl option \verb|ip_dynaddr| breaks 2686 this axiom. It has been made deliberately with the purpose 2687 of automatically reselecting the address on hosts with dynamic dial-out interfaces. 2688 However, this hack {\em must not\/} be used on multihomed hosts 2689 and especially on routers: it would break them. 2690\end{NB} 2691 2692 2693\item Otherwise, IP routing tables can contain an explicit source 2694address hint for this destination. The hint is set with the \verb|src| parameter 2695to the \verb|ip route| command, sec.\ref{IP-ROUTE}, p.\pageref{IP-ROUTE}. 2696 2697 2698\item Otherwise, the kernel searches through the list of addresses 2699attached to the interface through which the packets will be routed. 2700The search strategies are different for IP and IPv6. Namely: 2701 2702\begin{itemize} 2703\item IPv6 searches for the first valid, not deprecated address 2704with the same scope as the destination. 2705 2706\item IP searches for the first valid address with a scope wider 2707than the scope of the destination but it prefers addresses 2708which fall to the same subnet as the nexthop of the route 2709to the destination. Unlike IPv6, the scopes of IPv4 destinations 2710are not encoded in their addresses but are supplied 2711in routing tables instead (the \verb|scope| parameter to the \verb|ip route| command, 2712sec.\ref{IP-ROUTE}, p.\pageref{IP-ROUTE}). 2713 2714\end{itemize} 2715 2716 2717\item Otherwise, if the scope of the destination is \verb|link| or \verb|host|, 2718the algorithm fails and returns a zero source address. 2719 2720\item Otherwise, all interfaces are scanned to search for an address 2721with an appropriate scope. The loopback device \verb|lo| is always the first 2722in the search list, so that if an address with global scope (not 127.0.0.1!) 2723is configured on loopback, it is always preferred. 2724 2725\end{itemize} 2726 2727 2728\section{Proxy ARP/NDISC} 2729\label{PROXY-NEIGH} 2730 2731Routers may answer ARP/NDISC solicitations on behalf of other hosts. 2732In Linux-2.2 proxy ARP on an interface may be enabled 2733by setting the kernel \verb|sysctl| variable 2734\verb|/proc/sys/net/ipv4/conf/<dev>/proxy_arp| to 1. After this, the router 2735starts to answer ARP requests on the interface \verb|<dev>|, provided 2736the route to the requested destination does {\em not\/} go back via the same 2737device. 2738 2739The variable \verb|/proc/sys/net/ipv4/conf/all/proxy_arp| enables proxy 2740ARP on all the IP devices. 2741 2742However, this approach fails in the case of IPv6 because the router 2743must join the solicited node multicast address to listen for the corresponding 2744NDISC queries. It means that proxy NDISC is possible only on a per destination 2745basis. 2746 2747Logically, proxy ARP/NDISC is not a kernel task. It can easily be implemented 2748in user space. However, similar functionality was present in BSD kernels 2749and in Linux-2.0, so we have to preserve it at least to the extent that 2750is standardized in BSD. 2751\begin{NB} 2752 Linux-2.0 ARP had a feature called {\em subnet\/} proxy ARP. 2753 It is replaced with the sysctl flag in Linux-2.2. 2754\end{NB} 2755 2756 2757The \verb|ip| utility provides a way to manage proxy ARP/NDISC 2758with the \verb|ip neigh| command, namely: 2759\begin{verbatim} 2760 ip neigh add proxy ADDRESS [ dev NAME ] 2761\end{verbatim} 2762adds a new proxy ARP/NDISC record and 2763\begin{verbatim} 2764 ip neigh del proxy ADDRESS [ dev NAME ] 2765\end{verbatim} 2766deletes it. 2767 2768If the name of the device is not given, the router will answer solicitations 2769for address \verb|ADDRESS| on all devices, otherwise it will only serve 2770the device \verb|NAME|. Even if the proxy entry is created with 2771\verb|ip neigh|, the router {\em will not\/} answer a query if the route 2772to the destination goes back via the interface from which the solicitation 2773was received. 2774 2775It is important to emphasize that proxy entries have {\em no\/} 2776parameters other than these (IP/IPv6 address and optional device). 2777Particularly, the entry does not store any link layer address. 2778It always advertises the station address of the interface 2779on which it sends advertisements (i.e. it's own station address). 2780 2781\section{Route NAT status} 2782\label{ROUTE-NAT} 2783 2784NAT (or ``Network Address Translation'') remaps some parts 2785of the IP address space into other ones. Linux-2.2 route NAT is supposed 2786to be used to facilitate policy routing by rewriting addresses 2787to other routing domains or to help while renumbering sites 2788to another prefix. 2789 2790\paragraph{What it is not:} 2791It is necessary to emphasize that {\em it is not supposed\/} 2792to be used to compress address space or to split load. 2793This is not missing functionality but a design principle. 2794Route NAT is {\em stateless\/}. It does not hold any state 2795about translated sessions. This means that it handles any number 2796of sessions flawlessly. But it also means that it is {\em static\/}. 2797It cannot detect the moment when the last TCP client stops 2798using an address. For the same reason, it will not help to split 2799load between several servers. 2800\begin{NB} 2801It is a pretty commonly held belief that it is useful to split load between 2802several servers with NAT. This is a mistake. All you get from this 2803is the requirement that the router keep the state of all the TCP connections 2804going via it. Well, if the router is so powerful, run apache on it. 8) 2805\end{NB} 2806 2807The second feature: it does not touch packet payload, 2808does not try to ``improve'' broken protocols by looking 2809through its data and mangling it. It mangles IP addresses, 2810only IP addresses and nothing but IP addresses. 2811This also, is not missing any functionality. 2812 2813To resume: if you need to compress address space or keep 2814active FTP clients happy, your choice is not route NAT but masquerading, 2815port forwarding, NAPT etc. 2816\begin{NB} 2817By the way, you may also want to look at 2818http://www.suse.com/\~mha/HyperNews/get/linux-ip-nat.html 2819\end{NB} 2820 2821 2822\paragraph{How it works.} 2823Some part of the address space is reserved for dummy addresses 2824which will look for all the world like some host addresses 2825inside your network. No other hosts may use these addresses, 2826however other routers may also be configured to translate them. 2827\begin{NB} 2828A great advantage of route NAT is that it may be used not 2829only in stub networks but in environments with arbitrarily complicated 2830structure. It does not firewall, it {\em forwards.} 2831\end{NB} 2832These addresses are selected by the \verb|ip route| command 2833(sec.\ref{IP-ROUTE-ADD}, p.\pageref{IP-ROUTE-ADD}). F.e.\ 2834\begin{verbatim} 2835 ip route add nat 192.203.80.144 via 193.233.7.83 2836\end{verbatim} 2837states that the single address 192.203.80.144 is a dummy NAT address. 2838For all the world it looks like a host address inside our network. 2839For neighbouring hosts and routers it looks like the local address 2840of the translating router. The router answers ARP for it, advertises 2841this address as routed via it, {\em et al\/}. When the router 2842receives a packet destined for 192.203.80.144, it replaces 2843this address with 193.233.7.83 which is the address of some real 2844host and forwards the packet. If you need to remap 2845blocks of addresses, you may use a command like: 2846\begin{verbatim} 2847 ip route add nat 192.203.80.192/26 via 193.233.7.64 2848\end{verbatim} 2849This command will map a block of 63 addresses 192.203.80.192-255 to 2850193.233.7.64-127. 2851 2852When an internal host (193.233.7.83 in the example above) 2853sends something to the outer world and these packets are forwarded 2854by our router, it should translate the source address 193.233.7.83 2855into 192.203.80.144. This task is solved by setting a special 2856policy rule (sec.\ref{IP-RULE-ADD}, p.\pageref{IP-RULE-ADD}): 2857\begin{verbatim} 2858 ip rule add prio 320 from 193.233.7.83 nat 192.203.80.144 2859\end{verbatim} 2860This rule says that the source address 193.233.7.83 2861should be translated into 192.203.80.144 before forwarding. 2862It is important that the address after the \verb|nat| keyword 2863is some NAT address, declared by {\tt ip route add nat}. 2864If it is just a random address the router will not map to it. 2865\begin{NB} 2866The exception is when the address is a local address of this 2867router (or 0.0.0.0) and masquerading is configured in the linux-2.2 2868kernel. In this case the router will masquerade the packets as this address. 2869If 0.0.0.0 is selected, the result is equivalent to one 2870obtained with firewalling rules. Otherwise, you have the way 2871to order Linux to masquerade to this fixed address. 2872NAT mechanism used in linux-2.4 is more flexible than 2873masquerading, so that this feature has lost meaning and disabled. 2874\end{NB} 2875 2876If the network has non-trivial internal structure, it is 2877useful and even necessary to add rules disabling translation 2878when a packet does not leave this network. Let us return to the 2879example from sec.\ref{IP-RULE-SHOW} (p.\pageref{IP-RULE-SHOW}). 2880\begin{verbatim} 2881300: from 193.233.7.83 to 193.233.7.0/24 lookup main 2882310: from 193.233.7.83 to 192.203.80.0/24 lookup main 2883320: from 193.233.7.83 lookup inr.ruhep map-to 192.203.80.144 2884\end{verbatim} 2885This block of rules causes normal forwarding when 2886packets from 193.233.7.83 do not leave networks 193.233.7/24 2887and 192.203.80/24. Also, if the \verb|inr.ruhep| table does not 2888contain a route to the destination (which means that the routing 2889domain owning addresses from 192.203.80/24 is dead), no translation 2890will occur. Otherwise, the packets are translated. 2891 2892\paragraph{How to only translate selected ports:} 2893If you only want to translate selected ports (f.e.\ http) 2894and leave the rest intact, you may use \verb|ipchains| 2895to \verb|fwmark| a class of packets. 2896Suppose you did and all the packets from 193.233.7.83 2897destined for port 80 are marked with marker 0x1234 in input fwchain. 2898In this case you may replace rule \#320 with: 2899\begin{verbatim} 2900320: from 193.233.7.83 fwmark 1234 lookup main map-to 192.203.80.144 2901\end{verbatim} 2902and translation will only be enabled for outgoing http requests. 2903 2904\section{Example: minimal host setup} 2905\label{EXAMPLE-SETUP} 2906 2907The following script gives an example of a fault safe 2908setup of IP (and IPv6, if it is compiled into the kernel) 2909in the common case of a node attached to a single broadcast 2910network. A more advanced script, which may be used both on multihomed 2911hosts and on routers, is described in the following 2912section. 2913 2914The utilities used in the script may be found in the 2915directory ftp://ftp.inr.ac.ru/ip-routing/: 2916\begin{enumerate} 2917\item \verb|ip| --- package \verb|iproute2|. 2918\item \verb|arping| --- package \verb|iputils|. 2919\item \verb|rdisc| --- package \verb|iputils|. 2920\end{enumerate} 2921\begin{NB} 2922It also refers to a DHCP client, \verb|dhcpcd|. I should refrain from 2923recommending a good DHCP client to use. All that I can 2924say is that ISC \verb|dhcp-2.0b1pl6| patched with the patch that 2925can be found in the \verb|dhcp.bootp.rarp| subdirectory of 2926the same ftp site {\em does\/} work, 2927at least on Ethernet and Token Ring. 2928\end{NB} 2929 2930\begin{verbatim} 2931#! /bin/bash 2932\end{verbatim} 2933\begin{flushleft} 2934\# {\bf Usage: \verb|ifone ADDRESS[/PREFIX-LENGTH] [DEVICE]|}\\ 2935\# {\bf Parameters:}\\ 2936\# \$1 --- Static IP address, optionally followed by prefix length.\\ 2937\# \$2 --- Device name. If it is missing, \verb|eth0| is asssumed.\\ 2938\# F.e. \verb|ifone 193.233.7.90| 2939\end{flushleft} 2940\begin{verbatim} 2941dev=$2 2942: ${dev:=eth0} 2943ipaddr= 2944\end{verbatim} 2945\# Parse IP address, splitting prefix length. 2946\begin{verbatim} 2947if [ "$1" != "" ]; then 2948 ipaddr=${1%/*} 2949 if [ "$1" != "$ipaddr" ]; then 2950 pfxlen=${1#*/} 2951 fi 2952 : ${pfxlen:=24} 2953fi 2954pfx="${ipaddr}/${pfxlen}" 2955\end{verbatim} 2956 2957\begin{flushleft} 2958\# {\bf Step 0} --- enable loopback.\\ 2959\#\\ 2960\# This step is necessary on any networked box before attempt\\ 2961\# to configure any other device.\\ 2962\end{flushleft} 2963\begin{verbatim} 2964ip link set up dev lo 2965ip addr add 127.0.0.1/8 dev lo brd + scope host 2966\end{verbatim} 2967\begin{flushleft} 2968\# IPv6 autoconfigure themself on loopback.\\ 2969\#\\ 2970\# If user gave loopback as device, we add the address as alias and exit. 2971\end{flushleft} 2972\begin{verbatim} 2973if [ "$dev" = "lo" ]; then 2974 if [ "$ipaddr" != "" -a "$ipaddr" != "127.0.0.1" ]; then 2975 ip address add $ipaddr dev $dev 2976 exit $? 2977 fi 2978 exit 0 2979fi 2980\end{verbatim} 2981 2982\noindent\# {\bf Step 1} --- enable device \verb|$dev| 2983 2984\begin{verbatim} 2985if ! ip link set up dev $dev ; then 2986 echo "Cannot enable interface $dev. Aborting." 1>&2 2987 exit 1 2988fi 2989\end{verbatim} 2990\begin{flushleft} 2991\# The interface is \verb|UP|. IPv6 started stateless autoconfiguration itself,\\ 2992\# and its configuration finishes here. However,\\ 2993\# IP still needs some static preconfigured address. 2994\end{flushleft} 2995\begin{verbatim} 2996if [ "$ipaddr" = "" ]; then 2997 echo "No address for $dev is configured, trying DHCP..." 1>&2 2998 dhcpcd 2999 exit $? 3000fi 3001\end{verbatim} 3002 3003\begin{flushleft} 3004\# {\bf Step 2} --- IP Duplicate Address Detection~\cite{RFC-DHCP}.\\ 3005\# Send two probes and wait for result for 3 seconds.\\ 3006\# If the interface opens slower f.e.\ due to long media detection,\\ 3007\# you want to increase the timeout.\\ 3008\end{flushleft} 3009\begin{verbatim} 3010if ! arping -q -c 2 -w 3 -D -I $dev $ipaddr ; then 3011 echo "Address $ipaddr is busy, trying DHCP..." 1>&2 3012 dhcpcd 3013 exit $? 3014fi 3015\end{verbatim} 3016\begin{flushleft} 3017\# OK, the address is unique, we may add it on the interface.\\ 3018\#\\ 3019\# {\bf Step 3} --- Configure the address on the interface. 3020\end{flushleft} 3021 3022\begin{verbatim} 3023if ! ip address add $pfx brd + dev $dev; then 3024 echo "Failed to add $pfx on $dev, trying DHCP..." 1>&2 3025 dhcpcd 3026 exit $? 3027fi 3028\end{verbatim} 3029 3030\noindent\# {\bf Step 4} --- Announce our presence on the link. 3031\begin{verbatim} 3032arping -A -c 1 -I $dev $ipaddr 3033noarp=$? 3034( sleep 2; 3035 arping -U -c 1 -I $dev $ipaddr ) >& /dev/null </dev/null & 3036\end{verbatim} 3037 3038\begin{flushleft} 3039\# {\bf Step 5} (optional) --- Add some control routes.\\ 3040\#\\ 3041\# 1. Prohibit link local multicast addresses.\\ 3042\# 2. Prohibit link local (alias, limited) broadcast.\\ 3043\# 3. Add default multicast route. 3044\end{flushleft} 3045\begin{verbatim} 3046ip route add unreachable 224.0.0.0/24 3047ip route add unreachable 255.255.255.255 3048if [ `ip link ls $dev | grep -c MULTICAST` -ge 1 ]; then 3049 ip route add 224.0.0.0/4 dev $dev scope global 3050fi 3051\end{verbatim} 3052 3053\begin{flushleft} 3054\# {\bf Step 6} --- Add fallback default route with huge metric.\\ 3055\# If a proxy ARP server is present on the interface, we will be\\ 3056\# able to talk to all the Internet without further configuration.\\ 3057\# It is not so cheap though and we still hope that this route\\ 3058\# will be overridden by more correct one by rdisc.\\ 3059\# Do not make this step if the device is not ARPable,\\ 3060\# because dead nexthop detection does not work on them. 3061\end{flushleft} 3062\begin{verbatim} 3063if [ "$noarp" = "0" ]; then 3064 ip ro add default dev $dev metric 30000 scope global 3065fi 3066\end{verbatim} 3067 3068\begin{flushleft} 3069\# {\bf Step 7} --- Restart router discovery and exit. 3070\end{flushleft} 3071\begin{verbatim} 3072killall -HUP rdisc || rdisc -fs 3073exit 0 3074\end{verbatim} 3075 3076 3077\section{Example: {\protect\tt ifcfg} --- interface address management} 3078\label{EXAMPLE-IFCFG} 3079 3080This is a simplistic script replacing one option of \verb|ifconfig|, 3081namely, IP address management. It not only adds 3082addresses, but also carries out Duplicate Address Detection~\cite{RFC-DHCP}, 3083sends unsolicited ARP to update the caches of other hosts sharing 3084the interface, adds some control routes and restarts Router Discovery 3085when it is necessary. 3086 3087I strongly recommend using it {\em instead\/} of \verb|ifconfig| both 3088on hosts and on routers. 3089 3090\begin{verbatim} 3091#! /bin/bash 3092\end{verbatim} 3093\begin{flushleft} 3094\# {\bf Usage: \verb?ifcfg DEVICE[:ALIAS] [add|del] ADDRESS[/LENGTH] [PEER]?}\\ 3095\# {\bf Parameters:}\\ 3096\# ---Device name. It may have alias suffix, separated by colon.\\ 3097\# ---Command: add, delete or stop.\\ 3098\# ---IP address, optionally followed by prefix length.\\ 3099\# ---Optional peer address for pointopoint interfaces.\\ 3100\# F.e. \verb|ifcfg eth0 193.233.7.90/24| 3101 3102\noindent\# This function determines, whether it is router or host.\\ 3103\# It returns 0, if the host is apparently not router. 3104\end{flushleft} 3105\begin{verbatim} 3106CheckForwarding () { 3107 local sbase fwd 3108 sbase=/proc/sys/net/ipv4/conf 3109 fwd=0 3110 if [ -d $sbase ]; then 3111 for dir in $sbase/*/forwarding; do 3112 fwd=$[$fwd + `cat $dir`] 3113 done 3114 else 3115 fwd=2 3116 fi 3117 return $fwd 3118} 3119\end{verbatim} 3120\begin{flushleft} 3121\# This function restarts Router Discovery.\\ 3122\end{flushleft} 3123\begin{verbatim} 3124RestartRDISC () { 3125 killall -HUP rdisc || rdisc -fs 3126} 3127\end{verbatim} 3128\begin{flushleft} 3129\# Calculate ABC "natural" mask length\\ 3130\# Arg: \$1 = dotquad address 3131\end{flushleft} 3132\begin{verbatim} 3133ABCMaskLen () { 3134 local class; 3135 class=${1%%.*} 3136 if [ $class -eq 0 -o $class -ge 224 ]; then return 0 3137 elif [ $class -ge 192 ]; then return 24 3138 elif [ $class -ge 128 ]; then return 16 3139 else return 8 ; fi 3140} 3141\end{verbatim} 3142 3143 3144\begin{flushleft} 3145\# {\bf MAIN()}\\ 3146\#\\ 3147\# Strip alias suffix separated by colon. 3148\end{flushleft} 3149\begin{verbatim} 3150label="label $1" 3151ldev=$1 3152dev=${1%:*} 3153if [ "$dev" = "" -o "$1" = "help" ]; then 3154 echo "Usage: ifcfg DEV [[add|del [ADDR[/LEN]] [PEER] | stop]" 1>&2 3155 echo " add - add new address" 1>&2 3156 echo " del - delete address" 1>&2 3157 echo " stop - completely disable IP" 1>&2 3158 exit 1 3159fi 3160shift 3161 3162CheckForwarding 3163fwd=$? 3164\end{verbatim} 3165\begin{flushleft} 3166\# Parse command. If it is ``stop'', flush and exit. 3167\end{flushleft} 3168\begin{verbatim} 3169deleting=0 3170case "$1" in 3171add) shift ;; 3172stop) 3173 if [ "$ldev" != "$dev" ]; then 3174 echo "Cannot stop alias $ldev" 1>&2 3175 exit 1; 3176 fi 3177 ip -4 addr flush dev $dev $label || exit 1 3178 if [ $fwd -eq 0 ]; then RestartRDISC; fi 3179 exit 0 ;; 3180del*) 3181 deleting=1; shift ;; 3182*) 3183esac 3184\end{verbatim} 3185\begin{flushleft} 3186\# Parse prefix, split prefix length, separated by slash. 3187\end{flushleft} 3188\begin{verbatim} 3189ipaddr= 3190pfxlen= 3191if [ "$1" != "" ]; then 3192 ipaddr=${1%/*} 3193 if [ "$1" != "$ipaddr" ]; then 3194 pfxlen=${1#*/} 3195 fi 3196 if [ "$ipaddr" = "" ]; then 3197 echo "$1 is bad IP address." 1>&2 3198 exit 1 3199 fi 3200fi 3201shift 3202\end{verbatim} 3203\begin{flushleft} 3204\# If peer address is present, prefix length is 32.\\ 3205\# Otherwise, if prefix length was not given, guess it. 3206\end{flushleft} 3207\begin{verbatim} 3208peer=$1 3209if [ "$peer" != "" ]; then 3210 if [ "$pfxlen" != "" -a "$pfxlen" != "32" ]; then 3211 echo "Peer address with non-trivial netmask." 1>&2 3212 exit 1 3213 fi 3214 pfx="$ipaddr peer $peer" 3215else 3216 if [ "$pfxlen" = "" ]; then 3217 ABCMaskLen $ipaddr 3218 pfxlen=$? 3219 fi 3220 pfx="$ipaddr/$pfxlen" 3221fi 3222if [ "$ldev" = "$dev" -a "$ipaddr" != "" ]; then 3223 label= 3224fi 3225\end{verbatim} 3226\begin{flushleft} 3227\# If deletion was requested, delete the address and restart RDISC 3228\end{flushleft} 3229\begin{verbatim} 3230if [ $deleting -ne 0 ]; then 3231 ip addr del $pfx dev $dev $label || exit 1 3232 if [ $fwd -eq 0 ]; then RestartRDISC; fi 3233 exit 0 3234fi 3235\end{verbatim} 3236\begin{flushleft} 3237\# Start interface initialization.\\ 3238\#\\ 3239\# {\bf Step 0} --- enable device \verb|$dev| 3240\end{flushleft} 3241\begin{verbatim} 3242if ! ip link set up dev $dev ; then 3243 echo "Error: cannot enable interface $dev." 1>&2 3244 exit 1 3245fi 3246if [ "$ipaddr" = "" ]; then exit 0; fi 3247\end{verbatim} 3248\begin{flushleft} 3249\# {\bf Step 1} --- IP Duplicate Address Detection~\cite{RFC-DHCP}.\\ 3250\# Send two probes and wait for result for 3 seconds.\\ 3251\# If the interface opens slower f.e.\ due to long media detection,\\ 3252\# you want to increase the timeout.\\ 3253\end{flushleft} 3254\begin{verbatim} 3255if ! arping -q -c 2 -w 3 -D -I $dev $ipaddr ; then 3256 echo "Error: some host already uses address $ipaddr on $dev." 1>&2 3257 exit 1 3258fi 3259\end{verbatim} 3260\begin{flushleft} 3261\# OK, the address is unique. We may add it to the interface.\\ 3262\#\\ 3263\# {\bf Step 2} --- Configure the address on the interface. 3264\end{flushleft} 3265\begin{verbatim} 3266if ! ip address add $pfx brd + dev $dev $label; then 3267 echo "Error: failed to add $pfx on $dev." 1>&2 3268 exit 1 3269fi 3270\end{verbatim} 3271\noindent\# {\bf Step 3} --- Announce our presence on the link 3272\begin{verbatim} 3273arping -q -A -c 1 -I $dev $ipaddr 3274noarp=$? 3275( sleep 2 ; 3276 arping -q -U -c 1 -I $dev $ipaddr ) >& /dev/null </dev/null & 3277\end{verbatim} 3278\begin{flushleft} 3279\# {\bf Step 4} (optional) --- Add some control routes.\\ 3280\#\\ 3281\# 1. Prohibit link local multicast addresses.\\ 3282\# 2. Prohibit link local (alias, limited) broadcast.\\ 3283\# 3. Add default multicast route. 3284\end{flushleft} 3285\begin{verbatim} 3286ip route add unreachable 224.0.0.0/24 >& /dev/null 3287ip route add unreachable 255.255.255.255 >& /dev/null 3288if [ `ip link ls $dev | grep -c MULTICAST` -ge 1 ]; then 3289 ip route add 224.0.0.0/4 dev $dev scope global >& /dev/null 3290fi 3291\end{verbatim} 3292\begin{flushleft} 3293\# {\bf Step 5} --- Add fallback default route with huge metric.\\ 3294\# If a proxy ARP server is present on the interface, we will be\\ 3295\# able to talk to all the Internet without further configuration.\\ 3296\# Do not make this step on router or if the device is not ARPable.\\ 3297\# because dead nexthop detection does not work on them. 3298\end{flushleft} 3299\begin{verbatim} 3300if [ $fwd -eq 0 ]; then 3301 if [ $noarp -eq 0 ]; then 3302 ip ro append default dev $dev metric 30000 scope global 3303 elif [ "$peer" != "" ]; then 3304 if ping -q -c 2 -w 4 $peer ; then 3305 ip ro append default via $peer dev $dev metric 30001 3306 fi 3307 fi 3308 RestartRDISC 3309fi 3310 3311exit 0 3312\end{verbatim} 3313\begin{flushleft} 3314\# End of {\bf MAIN()} 3315\end{flushleft} 3316 3317 3318\end{document} 3319